Add GitLab auth token retrieval to the UI

[jpellizzari]

According to the ADR, the Gitlab auth flow for the frontend code/experience will need to go something like this: Note that this differs from the current GitHub auth strategy. It is still OAuth, but GitLab is a bit more involved.

1. Detect the provider from a URL that the user types in
2. (if Gitlab) Present the user with a button that will take them to the Gitlab OAuth page
3. After authorizing, Gitlab will send the user back to our callback page (JS or Go HTTP handler?)
4. A backend route will exchange the challenge code on the backend and covert it into a JWT that gets returned to the UI

Acceptance Criteria:

• A user should be able to complete the GitLab auth flow during adding/removing an app and listing commits
• Unit test coverage with sad path cases handled
• The GitLab UI auth flow should be re-usable by Enterprise (just like the GitHub one is today)

[josecordaz]

Testing in Chrome and Firefox.

We were able to see the JWT token in both browsers properly.

We noticed some messages referring to Github on Gitlab repos. We saw this message in the UI when trying to authenticate using the button on the commits section. Not sure if this should be expected in this PR.

And also seen this message after clicking on Remove App button

@jpellizzari Would you mind helping us to clarify if this is the expected behavior. If not no worries, we can to create some tickets to address these.

[palemtnrider]

While testing with the UI running from within the cluster, we are seeing an expired token error which prohibits adding an app.

@jpellizzari suspects we are requesting a token with an invalid expiration value.

CC @josecordaz

Log message from the container:

2021-11-29T23:07:22.344Z        INFO    middleware/middleware.go:93     invalid token format
[]
2021-11-29T23:07:23.445Z        INFO    middleware/middleware.go:93     invalid token format
2021-11-29T23:07:23.775Z        INFO    middleware/middleware.go:93     invalid token format
[]
2021-11-29T23:07:27.398Z        INFO    middleware/middleware.go:105    could not parse claims: token is expired by 4s: unauthorized token
2021-11-29T23:07:27.399Z        ERROR   middleware/middleware.go:40     server error    {"error": "rpc error: code = Unauthenticated desc = token error: could not get token from context"}
github.com/weaveworks/weave-gitops/pkg/server/middleware.WithGrpcErrorLogging.func1
        /app/pkg/server/middleware/middleware.go:40
github.com/grpc-ecosystem/grpc-gateway/v2/runtime.HTTPError
        /go/pkg/mod/github.com/grpc-ecosystem/grpc-gateway/v2@v2.5.0/runtime/errors.go:81
github.com/weaveworks/weave-gitops/pkg/api/applications.RegisterApplicationsHandlerServer.func11
        /app/pkg/api/applications/applications.pb.gw.go:959
github.com/grpc-ecosystem/grpc-gateway/v2/runtime.(*ServeMux).ServeHTTP
        /go/pkg/mod/github.com/grpc-ecosystem/grpc-gateway/v2@v2.5.0/runtime/mux.go:251
github.com/weaveworks/weave-gitops/pkg/server/middleware.WithLogging.func1
        /app/pkg/server/middleware/middleware.go:54
net/http.HandlerFunc.ServeHTTP
        /usr/local/go/src/net/http/server.go:2049
github.com/weaveworks/weave-gitops/pkg/server/middleware.WithProviderToken.func1
        /app/pkg/server/middleware/middleware.go:109
net/http.HandlerFunc.ServeHTTP
        /usr/local/go/src/net/http/server.go:2049
net/http.(*ServeMux).ServeHTTP
        /usr/local/go/src/net/http/server.go:2428
net/http.serverHandler.ServeHTTP
        /usr/local/go/src/net/http/server.go:2867
net/http.(*conn).serve
        /usr/local/go/src/net/http/server.go:1932