Proposal: expand media-storage cloud backends — Google Cloud Storage first

[regiscamimura]

Proposal

Add more cloud backends to the existing MediaStorage system, starting with Google Cloud
Storage. MediaStorage already ships Local and S3 backends; this adds GCSMediaStorage
as the next one — and I'm happy to follow with others (e.g. Azure Blob) after.

from piccolo_api.media.gcs import GCSMediaStorage

storage = GCSMediaStorage(
    column=Movie.poster,
    bucket_name="my-bucket",
    folder_name="movie_posters",
)

Design

Mirrors S3MediaStorage method-for-method (async + _sync worker via ThreadPoolExecutor,
reusing the base's generate_file_key), using google-cloud-storage. Optional dependency as
a gcs extra (pip install 'piccolo_api[gcs]'), like s3.

No new framework — MediaStorage's six abstract methods are already the provider interface,
so each new backend is just one module + extra + docs.

One GCS doc note: v4 signing needs a private key. Locally that's a service-account JSON;
on GCP compute (Cloud Run) the ADC has no private key, so signing goes through the IAM
signBlob API (grant the runtime SA roles/iam.serviceAccountTokenCreator).

Questions before I PR

1. Happy for GCS to live in piccolo_api, extending the existing backends?
2. google-cloud-storage + a gcs extra — agreeable?
3. Interested in further providers after GCS? If so, I could factor a small shared
   CloudMediaStorage mixin once there's a second one to share it with.

The GCS backend is implemented and tested (faked storage.Client, no live GCS) — happy to
open the PR once you confirm the direction. Thanks!

[dantownsend]

Yeah, it would be nice to have.

We had a PR a while ago, which added support for Azure object storage.

I feel bad because I didn't have time to fully test it, so it didn't get merged in.

I didn't use Azure at the time, so it was hard for me to test.

It would be nice to have support for storage other than S3 though. If we can do it in a clean way, I think it's a good idea.