Backport of auth/ldap: add username_as_alias configurable into release/1.8.x

[jasonodonnell]

Backport

Backport of #14324 1.8.

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

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This adds a new config parameter to the LDAP util SDK username_as_alias. A change in #11000 resulted in alias names changing, which has broken a number of users in 1.9+. Using this parameter, an operator can change alias names back.

For example, if LDAP auth was configured with no binddn or bindpass, in 1.8.8 the alias name would be:

[~] vault read identity/alias/id/66c3a58f-9a72-26cf-f5a9-438528398eb3
Key                          Value
---                          -----
...
mount_accessor               auth_ldap_6af4105c
mount_path                   auth/ldap/
mount_type                   ldap
name                         bob
namespace_id                 root

However, in 1.9.3, the same config would result in alias names like this:

[~] vault read identity/alias/id/67656061-c8d2-8379-a7a5-dfa2c89659c9
Key                          Value
---                          -----
...
mount_accessor               auth_ldap_01e39e22
mount_path                   auth/ldap/
mount_type                   ldap
name                         cn=bob,CN=Users,DC=corp,DC=example,DC=net
namespace_id                 root