[BE] Add PMM to version service

User story
To be able to update operators automatically or offer an update to the user, we need to be able to tell what operator can be installed without updating PMM. Let's start from 1.6.0 of PXC and PSMDB, previous versions were custom branches.
We can start from 1.8.0 for both operators because update feature will be present only in next release. We can add others later if use case arises.

Suggested implementation
We need to test what operators are compatible with what versions of PMM. Then add PMM to version service https://github.com/Percona-Lab/percona-version-service

See PR https://github.com/Percona-Lab/percona-version-service/pull/37/files and another product pmm-server to be able to answer questions (requests) like:
https://perconacorp.slack.com/archives/C01F3TX1CL8/p1622210248043500?thread_ts=1622209272.041900&cid=C01F3TX1CL8

Out of the scope

• updating operators
• querying version service to check for updates
• version return based on kubernetes provider and kubernetes version