[API, CLI] Enabling PMM UI to connect to Postgres using client certificates (for Remote Add)

User story:
As a PMM use, I want to be able to pass custom SSL keys to be able to connect to my remote PostgreSQL server, so that PMM can monitor it

UI/UX:

1. >pmm-admin add postgresql .. --ssl...

Acceptance criteria

• user able to pass PG related SSL keys:
  • sslcert
  • sslkey
  • sslrootcert
• Documentation:
  • updated documentation for UI of adding remote Pg 
  • BD

Out of scope:

Suggested implementation:
TBD

How to test:
TBD

Original report:

Can we include certificate based login for postgres monitoring? In UI can we add option to select sslmode and path to certificates and keys? we have requirement where all postgres will be ssl enabled and key and certificates will be used for authenetication

Sample connection string:

DATA_SOURCE_NAME=“postgresql://postgres_exporter:[email protected]:5432/postgres?sslmode=verify-ca&sslrootcert=/etc/ssl/certs/server-ca.pem&sslcert=/etc/ssl/certs/client-cert.pem&sslkey=/etc/ssl/certs/client-key.pem”

 Sample connection setting from grafana where you can see the certificate related entries