Support opening repository in read-only mode (#4302)

Useful for running read only operations against the repository

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [x] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/conn.go

@@ -184,6 +184,7 @@ func (w *conn) commonConnect(
 		ClientOptions: repo.ClientOptions{
 			Username: opts.User,
 			Hostname: opts.Host,
+			ReadOnly: opts.ReadOnly,
 		},
 	}
 

src/pkg/control/repository/repo.go

@@ -11,6 +11,7 @@ type Options struct {
 	// ViewTimestamp is the time at which the repo should be opened at if
 	// immutable backups are being used. If nil then the current time is used.
 	ViewTimestamp *time.Time `json:"viewTimestamp"`
+	ReadOnly      bool       `json:"readonly,omitempty"`
 }
 
 type Maintenance struct {

src/pkg/repository/repository_test.go

@@ -393,6 +393,31 @@ func (suite *RepositoryIntegrationSuite) TestConnect_DisableMetrics() {
 	assert.Equal(t, r.GetID(), r.GetID())
 }
 
+func (suite *RepositoryIntegrationSuite) TestConnect_ReadOnly() {
+	t := suite.T()
+
+	ctx, flush := tester.NewContext(t)
+	defer flush()
+
+	// need to initialize the repository before we can test connecting to it.
+	st := storeTD.NewPrefixedS3Storage(t)
+
+	repo, err := Initialize(
+		ctx,
+		account.Account{},
+		st,
+		control.DefaultOptions(),
+		ctrlRepo.Retention{})
+	require.NoError(t, err)
+
+	// now re-connect
+	r, err := Connect(ctx, account.Account{}, st, repo.GetID(), control.Options{Repo: ctrlRepo.Options{ReadOnly: true}})
+	assert.NoError(t, err)
+
+	// now we have repoID beforehand
+	assert.Equal(t, r.GetID(), r.GetID())
+}
+
 // Test_Options tests that the options are passed through to the repository
 // correctly
 func (suite *RepositoryIntegrationSuite) Test_Options() {