feat: port "create-cephfs-client" from charm.ceph

[jedel1043]

Description

Applies changes:

• broker: add operation to create CephFS clients
• test: add unit testing for new "create-cephfs-client" op

with some modifications to adapt it to the charm-microceph codebase.

This is separate from #68, since it allows client charms to create cephfs clients using broker requests, while the previous PR only allows creating the filesystem itself.

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

A new unit test on the broker tests file.

Contributor's Checklist

Please check that you have:

• [x] self-reviewed the code in this PR.
• [x] added code comments, particularly in hard-to-understand areas.
• [x] updated the user documentation with corresponding changes.
• [x] added tests to verify effectiveness of this change.

[UtkarshBhatthere]

/canonical/self-hosted-runners/run-workflows ca5ea94ec108a1340f435f96cd4fcc2654126a70