Add S3 remote storage for snapshot save, load, and list

[anisaoshafi]

What

Adds support for saving, loading, and listing snapshots in your own AWS S3 bucket. Before this, a snapshot could only go to a local file or a LocalStack platform pod (pod:).

How

Set your AWS credentials, then use an s3:// path:

export AWS_ACCESS_KEY_ID=...
export AWS_SECRET_ACCESS_KEY=...

lstk snapshot save my-pod s3://my-bucket/prefix    # save to S3
lstk snapshot load my-pod s3://my-bucket/prefix    # load it back
lstk snapshot list s3://my-bucket/prefix           # list what's in the bucket

Or use a named AWS profile instead of env vars:

lstk snapshot save my-pod s3://my-bucket/prefix --profile myprofile
lstk snapshot load my-pod s3://my-bucket/prefix --profile myprofile
lstk snapshot list s3://my-bucket/prefix --profile myprofile

• The pod name (my-pod) is the snapshot's name in the bucket. Required for load; auto-generated for save if you leave it out.
• Credentials come from AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY (and optional AWS_SESSION_TOKEN), or from --profile.
• Credentials can't be put in the URL.

Under the hood

• lstk doesn't talk to S3 itself. It passes your credentials to the emulator, and the emulator does the upload and download. No AWS SDK was added to lstk.
• Credentials aren't stored. lstk registers the bucket with a placeholder URL and sends the real values with each command.
• snapshot list s3://... needs a running emulator, since the emulator is what reads the bucket.