Ability to set emulator host/port from .setHost rather than env var

[saturnism]

Environment details

1. OSX, Java

Steps to reproduce

1. To use Testcontainer + Firestore emulator is extremely difficult.

Code example

Firestore client library can only connect to a firestore emulator by using an env var. This is not idiomatic for Java unit tests.

it's impossible to set the emulator host/port simply from .setHost(..)

FirestoreOptions firestoreOptions =
   FirestoreOptions.newBuilder().setHost(...).

The real code is this:

// duplicate the FakeCredentials class because it's package protected and not publicly accessible.
static class FakeCredentials extends Credentials {
 private final Map<String, List<String>> HEADERS =
     ImmutableMap.of("Authorization", Arrays.asList("Bearer owner"));
...
}

// then, duplicate the code that otherwise executes from an env var:
FirestoreOptions firestoreOptions =
   FirestoreOptions.newBuilder()
       .setProjectId(FIRESTORE_PROJECT_ID)
       .setChannelProvider(
           InstantiatingGrpcChannelProvider.newBuilder()
               .setEndpoint(
                   String.format(
                       "%s:%d",
                       FIRESTORE_CONTAINER.getContainerIpAddress(),
                       FIRESTORE_CONTAINER.getMappedPort(8080)))
               .setChannelConfigurator(
                   (managedChannelBuilder -> managedChannelBuilder.usePlaintext()))
               .build())
       .setCredentialsProvider(FixedCredentialsProvider.create(new FakeCredentials()))
       .build();

Any additional information below

It'd be MUCH easier if emulator host/port can simply be set via .setHost(...), OR, a convenience method, such as .useEmulator(host, port)

[saturnism]

/cc @eddumelendez who's also working with TestContainer integration for GCP. https://github.com/eddumelendez/testcontainers-java/tree/gcloud

[saturnism]

hmm, i think this is the wrong repo after filing the issue. moving to googleapis/java-firestore#210