Add support for "Server" and "x-noobaa-available-storage-classes" headers

[tangledbytes]

Explain the changes

This PR adds support for:

1. Server HTTP header whose value is constant and is sent as part of every response.
2. x-noobaa-available-storage-classes which is sent as part of HeadBucket response. The value could be any combination of storage classes GLACIER and STANDARD based on NooBaa config.

Issues: Fixed #xxx / Gap #xxx

Fixed #8229

• [ ] Doc added/updated
• [ ] Tests added