Add LDAPIdentityProvider.spec.groupSearch.userAttributeForFilter

[cfryanr]

Addresses #1533.

Release note:

Added new `LDAPIdentityProvider.spec.groupSearch.userAttributeForFilter` and 
`ActiveDirectoryIdentityProvider.spec.groupSearch.userAttributeForFilter` configuration options.
The additional flexibility for LDAP and AD group searches introduced by this new configuration option can
be used to find groups in new ways, such as finding groups defined using the `posixGroup` schema.
For backwards compatibility, the group search defaults to the old behavior when this new option is not set.
For more details, see the API documentation https://github.com/vmware-tanzu/pinniped/blob/main/generated/1.27/README.adoc#ldapidentityprovidergroupsearch.

[codecov]

Codecov Report

Merging #1534 (d004859) into main (33cc973) will increase coverage by 0.03%.
The diff coverage is 95.74%.

@@            Coverage Diff             @@
##             main    #1534      +/-   ##
==========================================
+ Coverage   75.39%   75.42%   +0.03%     
==========================================
  Files         165      165              
  Lines       14942    14968      +26     
==========================================
+ Hits        11265    11290      +25     
- Misses       3383     3385       +2     
+ Partials      294      293       -1     

Impacted Files	Coverage Δ
...onfig/ldapupstreamwatcher/ldap_upstream_watcher.go	75.86% <71.42%> (-0.90%)	⬇️
...streamwatcher/active_directory_upstream_watcher.go	89.59% <100.00%> (+0.11%)	⬆️
internal/upstreamldap/upstreamldap.go	86.13% <100.00%> (+0.53%)	⬆️

... and 1 file with indirect coverage changes

[cfryanr]

I manually tested the code committed so far on this WIP PR and it works.

Next, need to write some integration tests, probably in supervisor_login_test.go, which:

1. Needs to work when run locally after using prepare-for-integration-tests.sh.
2. Needs to work in CI on Kind clusters with the openldap server running in the same cluster.
3. Needs to work in CI when run against JumpCloud's LDAP, or needs to be skipped in that case.

Some small tests could also be added to ldap_client_test.go.

[cfryanr]

Integration tests added!