Error when adding Amazon RDS MySQL with TLS over API

User Impact:
User cannot add mysql RDS instance when tls: true property is presented.

Steps to Reproduce:

Go to swagger on your instance: [https://127.0.0.1/swagger#/RDS/AddRDS
]

{
  "engine": 1,
  "region": "us-east-1",
  "aws_access_key": "ACCESS KEY",
  "aws_secret_key": "SECRET KEY",
  "instance_id": "rds-mysql57",
  "address": "rds-mysql57.cmtxparzenzi.us-east-1.rds.amazonaws.com",
  "port": 3306,
  "node_name": "rds",
  "service_name": "rdsmysql",
  "username": "USER",
  "password": "PASSWORD",
  "skip_connection_check": false,
  "tls": true,
  "tls_skip_verify": true
}

Execute code above.

Actual Result:

{
  "error": "Connection check failed: invalid value / unknown config name: custom.",
  "code": 9,
  "message": "Connection check failed: invalid value / unknown config name: custom."
}

Expected Result:

Instance was added successfully.

Suggested implementation:

We need do some code changes in pmm-agent, pmm-managed part for SSL MySQL. 

https://github.com/percona/pmm-agent/pull/249/files#r624931726

https://github.com/percona/pmm-managed/pull/720#discussion_r624992455

 How to test:
1. Use new flags in pmm-admin to add instance (tls, tls-ca, tls-cert, tls-key). Use certificates from attachment. Bold ones are needed.
Example: ./bin/pmm-admin add mysql -username=fry --password=pass ***********tls* tls-ca=mysql/ca.pem tls-cert=mysql/cert.pem -tls-key=mysql/key.pem --server-url=http://admin:[email protected] --tls-skip-verify --query-source=perfschema mysqlssl
Cases:
--tls
--tls --tls-skip-verify
--tls --skip-connection-check
--tls --tls-skip-verify  --skip-connection-check

2. Go to PMM inventory and check if agent running and tls: true property is presented.
3. Check if you getting data in instance summary and query analytics, also check all possible actions for mysql.
4. Now try this in way written in Steps to reproduce section.

Original report: https://perconacorp.slack.com/archives/C03J8FZFU/p1621422676030900