[pgAdmin4][Patch] - RM 2186 - Support external authentication sources [LDAP]

Hi,

Please find the attached patch to support LDAP Authentication in Server
mode.
To test the patch, config_auth.py needs to be configured for LDAP
configurations. The config settings are explained in this file in detail.
After configuring the parameters, start the pgadmin server in Server mode
and connect with LDAP server with the valid user via login page.

I have tested this patch with ldap and ldap + ssl/tls. With the TLS, I have
used the default config of ldap3 without certificates.

@Dave, can you please review this patch, as you have a better understanding
of LDAP and you can easily pointed out if I have missed anything.

Note: For the document update I will create the task and assign to Nidhi
for the same.

Thanks,
Khushboo