Implement user search in BE for CCSM

Context:

To enable collection user management in CCSM, the BE needs to implement user search using identity. 

Since identity user search may not always be available (depending on new identity feature implementation scope: https://github.com/camunda-cloud/identity/issues/2162), we also need a new UIConfiguration flag to signal to the FE whether or not user search is available. Set this according to identity.users().available(). If identity.users().available() is not available by the time of implementation, default to true for now and create follow up to adjust.

Note limitation: identity does not yet have a group search so this will only include users.

AT:

• IdentityRestService search API can retrieve users for a given search term as used in the collections user modal in the same way as c7/saas
• double check that helm charts always set the baseURL (required for user search) - I think this was already implemented with multi tenancy. If not, open a follow up ticket
• Add a new flag in UIConfiguration that is true if user search is available and false if it is not available. This depends on identity. Whether or not user search is available in identity can be checked with identity.users().available() – see slack link in comments. Not available yet – either identity implement this before this feature is ready in which case we can use it, or we need a follow up.
• UIConfiguration rest docs are updated

Dependencies:

Depends on: https://github.com/camunda-cloud/identity/issues/2278

Soft depends on: identity.users().available():  https://github.com/camunda-cloud/identity/issues/2304  - follow up here: OPT-7412
Influenced by https://github.com/camunda-cloud/identity/issues/2162

Testing notes:

To be tested with FE ticket: OPT-7404