Specify Custom Basic Auth password for Agents when adding Services

User story
As a PMM user, I want to be able to specify a custom password for accessing metrics from exporters, so that I can simplify additional data collection by external Prometheus

UI/UX:
pmm-admin add ... --agent-password='mypass' ..

Acceptance criteria

• User able to specify a custom password for BAsic Auth used to access the metrics endpoint
• metrics collection for PMM works the same as with the current passwords schema (agent Id)
• custom password exposed in Inventory UI/API so the user is able to differentiate between custom and standard passwords

Out of scope:

• validation for custom password
• setting a password for HAProxy monitoring or External Services monitoring 

Suggested implementation:
1. Add a new flag to all commands in pmm-admin

2. Extend API

3. In pmm-managed add migration. Store and select new pass from DB and pass it to pmm-agent

4. Add tests

How to test:

1. Run PMM server.
2. Run MySQL, ProxySQL, MongoDB and PostgreSQL.
3. Add all of them by pmm-admin command add with flag --agent-password which setup new password for endpoint /metrics instead agent ID. User is still pmm.
4. Check that everyting is working properly.
5. Check /metrics endpoint for all exporters and log in with new password.

Details:
Should be tested for all services

UPD. Agent password shouldn't be exposed on UI