Add GCSSettings to make endpointUrl + basePath configurable

[mdedetrich]

This PR adds a GCSSettings for google-cloud-stroage in order to give the ability to configure the host and the basePath. This is mainly done due to the fact that its currently not possible to test GCS using an implementation such as https://github.com/fsouza/fake-gcs-server since the endpoints are hardcoded at

alpakka/google-cloud-storage/src/main/scala/akka/stream/alpakka/googlecloud/storage/impl/GCStorageStream.scala

Lines 33 to 34 in f2971ca

	private val baseUrl = "https://storage.googleapis.com/"
	private val basePath = "/storage/v1"

.

Of note is there was a previous GCStorageSettings which was deprecated beforehand since there are a lot of settings that are shared between Google services. In order to not create conflicts/confusion, a seperate GCSSettings was created and it contains settings that are only specific to Google Cloud Storage so from a design PoV there shouldn't be any issues (common settings go into GoogleSettings, google service specific settings i.e. endpoints for a specific service go into the projects specific settings).

Tests were also made to make sure that the settings were correctly loaded. Afaik no binary incompatible changes were introduced.

If this PR is accepted/merged it means its also possible to use https://github.com/fsouza/fake-gcs-server to test GCS in alpakka in a similra fashion to how S3 is being tested with https://min.io/ (currently the tests just an in memory mock which is arguably not ideal).

[ennru]

Thank you for keeping up with all that config ceremony.

[mdedetrich]

@ennru I have just done as request, the main config is now under alpakka.google.cloud-storage and I have fixed the old class reference.

Also adjusted the commit message so its shorter, let me know if anything else is needed.

[ennru]

LGTM.

[ennru]

Thank you!