fix(fcm): Add ability to override default FCM endpoint via ClientOptions

[kl177]

Fixes #189

[hiranya911]

Thanks @slirx for the PR. We do not want to introduce an env variable for this purpose. Ideally we should be able to use the endpoint set via options.WithEndpoint(). This is already returned by the call to transport.NewHTTPClient(), and is currently ignored.

[kl177]

I've updated it to be able to use the endpoint set via options.WithEndpoint().
I've used internaloption.WithDefaultEndpoint() in order to apply the default endpoint.

[hiranya911]

Suggested one change. Also require some tests.

[hiranya911]

Thanks @slirx. LGTM with some minor nits. I'll merge this when those points are addressed.

[hiranya911]

Only WithEndpoint() should be sufficient here.

[kl177]

It gives an error only with WithEndpoint():

app, err := NewApp(ctx, nil, option.WithEndpoint(ts.URL))
if err != nil {
    t.Fatal(err)
}

c, err := app.Messaging(ctx)
if c == nil || err != nil {
    t.Fatalf("Messaging() = (%v, %v); want (iid, nil)", c, err)
}

--- FAIL: TestMessagingSendWithCustomEndpoint (0.59s)
    firebase_test.go:390: Messaging() = (<nil>, project ID is required to access Firebase Cloud Messaging client); want (iid, nil)

[hiranya911]

Ok, I suppose the transport still requires some sort of a credential. In that case let's keep option.WithTokenSource(). You definitely don't need WithCredentialsFile() when a TokenSource is explicitly provided.

[kl177]

It seems WithCredentialsFile() is required to provide project id. It gives an error without WithCredentialsFile():

tokenSource := &testTokenSource{AccessToken: "mock-token-from-custom"}
app, err := NewApp(
    ctx,
    nil,
    //option.WithCredentialsFile("testdata/service_account.json"),
    option.WithTokenSource(tokenSource),
    option.WithEndpoint(ts.URL),
)
if err != nil {
    t.Fatal(err)
}

c, err := app.Messaging(ctx)
if c == nil || err != nil {
    t.Fatalf("Messaging() = (%v, %v); want (iid, nil)", c, err)
}

--- FAIL: TestMessagingSendWithCustomEndpoint (0.00s)
    firebase_test.go:390: Messaging() = (<nil>, project ID is required to access Firebase Cloud Messaging client); want (iid, nil)

[hiranya911]

Ah! :(