[v2] Expose --bucket-name-prefix and --bucket-region to s3 ls

[hssyoo]

S3's ListBuckets API released filter parameters Prefix and BucketRegion. This PR exposes the low-level parameters to the high-level s3 ls command by exposing the --bucket-name-prefix and --bucket-region parameters. These parameters are only used if no S3 URI is supplied to the command:

• aws s3 ls --bucket-name-prefix foo is fine.
• aws s3 ls s3://mybucket --bucket-name-prefix foo is functionally the same as aws s3 ls s3://mybucket.

Decisions made:

• Map Prefix to --bucket-name-prefix. s3 ls calls ListObjectsV2 under the hood if an S3 URI is supplied. In that case, the bucket name and prefix/key are parsed, and the Prefix parameter is automatically (or implicitly) supplied. Without an S3 URI, ListBuckets is called and the Prefix parameter must be explicitly supplied by the high-level s3 ls parameter. To avoid confusion, ListBuckets's Prefix parameter is exposed as --bucket-name-prefix.
• The client doesn't attempt to validate that the 2 new parameters are only used when an S3 URI is not supplied. So aws s3 ls s3://mybucket --bucket-region us-east-1 does not throw an error/warning.
  1. This is consistent with how parameters are generally handled for high-level s3 commands.
  2. The parameter names clearly refer to buckets (not objects) and additional details are available in the help docs.

v1 PR will be opened after this is approved.

[aemous]

Looks good overall. I'd like to a see a unit & functional tests against specifying one of these args with an S3 URI, to verify it ignores the arg.