feat(graphQL): add ldap related apis

[zwpaper]

TODOs:

• list auth providers in graphQL
• starttls/ldaps support
• optimise error handling

for easy testing:

1. setup lldap by docker compose locally: https://github.com/lldap/lldap#with-docker
2. go to http://localhost:17070 to manage the local ldap
3. use testLdapCredential to test the configuration before add it to tabby
4. use updateLdapCredential to add the configuration
5. use tokenAuthLdap to login with LDAP
6. use deleteLdapCredential to delete the configuration

cc @liangfung