feat(GODT-2800): User Event Service

This patch adds the User Event Service which is meant to replace the
current event polling flow.

Each user interested in receiving events should register a new
subscriber using the `Service.Subscribe` function and then react on
the incoming events.

The current patch does not hook this up Bridge user as there are no
existing consumers, but it does provide extensive testing for the
expected behavior.

internal/services/userevents/mocks/mocks.go

@@ -0,0 +1,119 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/ProtonMail/proton-bridge/v3/internal/services/userevents (interfaces: EventSource,EventIDStore)
+
+// Package mocks is a generated GoMock package.
+package mocks
+
+import (
+	context "context"
+	reflect "reflect"
+
+	proton "github.com/ProtonMail/go-proton-api"
+	gomock "github.com/golang/mock/gomock"
+)
+
+// MockEventSource is a mock of EventSource interface.
+type MockEventSource struct {
+	ctrl     *gomock.Controller
+	recorder *MockEventSourceMockRecorder
+}
+
+// MockEventSourceMockRecorder is the mock recorder for MockEventSource.
+type MockEventSourceMockRecorder struct {
+	mock *MockEventSource
+}
+
+// NewMockEventSource creates a new mock instance.
+func NewMockEventSource(ctrl *gomock.Controller) *MockEventSource {
+	mock := &MockEventSource{ctrl: ctrl}
+	mock.recorder = &MockEventSourceMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockEventSource) EXPECT() *MockEventSourceMockRecorder {
+	return m.recorder
+}
+
+// GetEvent mocks base method.
+func (m *MockEventSource) GetEvent(arg0 context.Context, arg1 string) ([]proton.Event, bool, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetEvent", arg0, arg1)
+	ret0, _ := ret[0].([]proton.Event)
+	ret1, _ := ret[1].(bool)
+	ret2, _ := ret[2].(error)
+	return ret0, ret1, ret2
+}
+
+// GetEvent indicates an expected call of GetEvent.
+func (mr *MockEventSourceMockRecorder) GetEvent(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetEvent", reflect.TypeOf((*MockEventSource)(nil).GetEvent), arg0, arg1)
+}
+
+// GetLatestEventID mocks base method.
+func (m *MockEventSource) GetLatestEventID(arg0 context.Context) (string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetLatestEventID", arg0)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetLatestEventID indicates an expected call of GetLatestEventID.
+func (mr *MockEventSourceMockRecorder) GetLatestEventID(arg0 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetLatestEventID", reflect.TypeOf((*MockEventSource)(nil).GetLatestEventID), arg0)
+}
+
+// MockEventIDStore is a mock of EventIDStore interface.
+type MockEventIDStore struct {
+	ctrl     *gomock.Controller
+	recorder *MockEventIDStoreMockRecorder
+}
+
+// MockEventIDStoreMockRecorder is the mock recorder for MockEventIDStore.
+type MockEventIDStoreMockRecorder struct {
+	mock *MockEventIDStore
+}
+
+// NewMockEventIDStore creates a new mock instance.
+func NewMockEventIDStore(ctrl *gomock.Controller) *MockEventIDStore {
+	mock := &MockEventIDStore{ctrl: ctrl}
+	mock.recorder = &MockEventIDStoreMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockEventIDStore) EXPECT() *MockEventIDStoreMockRecorder {
+	return m.recorder
+}
+
+// Load mocks base method.
+func (m *MockEventIDStore) Load(arg0 context.Context) (string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Load", arg0)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Load indicates an expected call of Load.
+func (mr *MockEventIDStoreMockRecorder) Load(arg0 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Load", reflect.TypeOf((*MockEventIDStore)(nil).Load), arg0)
+}
+
+// Store mocks base method.
+func (m *MockEventIDStore) Store(arg0 context.Context, arg1 string) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Store", arg0, arg1)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// Store indicates an expected call of Store.
+func (mr *MockEventIDStoreMockRecorder) Store(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Store", reflect.TypeOf((*MockEventIDStore)(nil).Store), arg0, arg1)
+}