vfs_ceph_rgw: General fixes

This MR incorporates following changes (supersedes !4662 (closed), !4681 (closed) and !4716 (closed).).

1. Report ENOATTR error for extended attribute APIs, in case if they return ENOENT.

   • This is important to handle object with psuedo-path like "d1/d2/d3/obj1" where client wish to access file obj1,

     however, path d1/d2/d3 does not exists yet.

2. Make use of "how->flag" to identify write operation and use rgw_open/close APIs accordingly.
3. Add new module parameters to control number of threads per module instance.
4. Documentation changes

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code