HDDS-16672. Support RFC 2047 encoding for S3 user metadata

[F64116045]

What changes were proposed in this pull request?

The ceph/s3-tests test_object_set_get_unicode_metadata exposed that S3 Gateway does not handle non-ASCII user metadata correctly.

Investigation showed that S3 uses RFC 2047 for non-ASCII x-amz-meta-* values. Ozone currently stores encoded values as ordinary text and does not encode them when returning response headers.

This patch decodes RFC 2047 Q/B values before storing metadata and encodes non-ASCII values in GET and HEAD responses.

References:

• ceph/s3-tests failure case
• Amazon S3 metadata behavior

What is the link to the Apache JIRA?

https://issues.apache.org/jira/browse/HDDS-16672

How was this patch tested?

CI: https://github.com/F64116045/ozone/actions/runs/37118609154

Uploaded an object with this RFC 2047 encoded café metadata:

x-amz-meta-meta1: =?UTF-8?B?Y2Fmw6k=?=

The same request was tested against Ozone before and after this patch, and against S3.

B and Q are two RFC 2047 representations of the same café value.

	Ozone before	Ozone after	S3
Stored value	Encoded header text	café	Not observable
HEAD response	B-encoded café	Q-encoded café	Q-encoded café

[F64116045]

The original ceph/s3-tests case still fails because it sends raw non-ASCII metadata instead of RFC 2047. S3 does not round-trip that input either.