add a gateway endpoint for PutBucketTagging

[amwolff]

Add a gateway endpoint for PutBucketTagging. #549 must be done first. Make sure you add exhaustive tests. Useful context from @pwilloughby:

[…]

• A tag set can contain as many as 50 tags
• The tag key is a case-sensitive string that can contain 1 to 128 Unicode characters.
• The tag value is a case-sensitive string that can contain from 0 to 256 Unicode characters

There are no operations on individual tags. A whole tagset is read, deleted, written or overwritten with the get, delete, and put endpoints. The minio code uses the globalbucketmetadata stuff which will need to be removed and plumbed through the gateway, uplink, and satellite layers.

[…]

Originally posted by @pwilloughby in #7378