Add support for S3 Multi-Region Access Point (MRAP) URLs

[fedemengo]

Description

S3 Multi-Region Access Points (MRAPs) use ARN-based URLs instead of a plain bucket name:

s3://arn:aws:s3::123456789012:accesspoint/my-mrap.mrap/path/to/file.txt

Currently, S3Path parses the bucket by splitting on the first / after s3://, which gives arn:aws:s3::123456789012:accesspoint — cutting off the .mrap suffix. This causes bucket and key to be computed incorrectly, breaking all boto3 calls since they receive the wrong Bucket and Key parameters.

Current behavior

p = S3Path("s3://arn:aws:s3::123456789012:accesspoint/my-mrap.mrap/folder/file.txt")
p.bucket  # "arn:aws:s3::123456789012:accesspoint"  ← wrong
p.key     # "my-mrap.mrap/folder/file.txt"          ← wrong

Expected behavior

p.bucket  # "arn:aws:s3::123456789012:accesspoint/my-mrap.mrap"  ← full ARN
p.key     # "folder/file.txt"                                     ← correct

Proposed fix

Override the bucket property on S3Path to detect MRAP ARNs via a regex and return the full ARN when matched, falling back to the existing behaviour for regular bucket names.

[pjbull]

Thanks @fedemengo, we'd accept a PR with the proposed fix + tests. Check the contributing docs here:
https://cloudpathlib.drivendata.org/stable/contributing/

[fedemengo]

@pjbull awesome, I've opened #557