[java-firestore] Ability to set emulator host/port from .setHost rather than env var #13143

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

// 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();