Add matrix_synapse_ext_password_provider_ldap_tls_options_ca_certs_file
variable to allow specifying a custom CA certificate file for LDAP TLS
verification. Useful when Synapse is running in a container that does not
trust a private/internal CA by default.

Example usage:
matrix_synapse_ext_password_provider_ldap_tls_options_ca_certs_file: /etc/ssl/certs/my-ca.crt