Let pmm-agent have its own model and database table

Problem

Currently, pmm-agent is treated by PMM like an agent even though it is not. Also, it shares the data model with other agents. Such architecture led to a few issues, namely:

• pmm-agent database records are stored in the same table along with other agents

• there is a lot of places in the code where pmm-agent receives a special treatment compared to other, regular agents

This makes the code a bit complicated and difficult to evolve or migrate, for example p.3 of PMM-12915 is being blocked by this ticket.

Solution

• Define a separate data model dedicated just to pmm-agent

• Migrate pmm-agent records to a dedicated pmm_agents table

• Refactor the code to support these changes

• Fix the tests