Add SearchIndexing API methods to EnterpriseAdminClient

The following methods have been added for queuing an index job on their
target.

 #index_user
   alias #index_organization
 #index_repository
 #index_repository_issues
 #index_repository_code
 #index_users_repositories
   alias #index_organizations_repositories
 #index_users_repositories_issues
   alias #index_organizations_repositories_issues
 #index_users_repositories_code
   alias #index_organizations_repositories_code

Test cassettes recorded responses from GitHub Enterprise Server 11.10.327.