Backport of Add worker pool for LDAP group lookup into release/1.14.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #22659 to be assessed for backporting due to the inclusion of the label backport/1.14.x.

The below text is copied from the body of the original PR.

---

This PR aims to make LDAP group lookup more performant by adding a worker pool for group searches. Specifically this is helpful when use_token_groups=true and we need to look up every group individually in LDAP associated with the user.

In my testing where a LDAP user is a member of 300 LDAP groups and has 300 policies mapped to those groups I see significant performance increase by using a worker pool:

Before:

$ time vault login -method=ldap username=bob password=password
Success! You are now authenticated. The token information displayed below
is already stored in the token helper. You do NOT need to run "vault login"
again. Future Vault requests will automatically use this token.

Key                    Value
---                    -----
token                  hvs.CAESIN2lzMYdtMMQ31k5MoBccnS8exljazJc6ahv40RN9sfJGh4KHGh2cy5kSGFyS3diVXNPRmIwWG5pdkw4OWV2Sjg
token_accessor         rOEMnAWD9hUWMqUL1Md5ko84
token_duration         768h
token_renewable        true
...

real	0m16.168s
user	0m0.057s
sys	0m0.027s

After:

$ time vault login -method=ldap username=bob password=password
Success! You are now authenticated. The token information displayed below
is already stored in the token helper. You do NOT need to run "vault login"
again. Future Vault requests will automatically use this token.

Key                    Value
---                    -----
token                  hvs.CAESIE311YhuFnlcsN2BYJKc0dnOnMNMnXLRBOMxpaXcD1s5Gh4KHGh2cy5iZjc0dkx5ZjdHVFBySUxiUERmTGpMSXg
token_accessor         n8QwGvA5HDXIcfB77FaT3hye
token_duration         768h
token_renewable        true
...

real	0m2.303s
user	0m0.059s
sys	0m0.029s

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• 4e963c4

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