Summary:
this diff adds the probe manager to MDS. it is currently not hooked up yet--that is to come in a later diff.
the probe manager is responsible for handling any probe creation/updates and sending off registration messages to agents over NATS.

the plan is to have a reference to the probe manager in metadata/server.go, which receives RPC requests creating/checking probe status.
there will also be a reference to probe manager in the agent topic listener, where we receive probe update messages.

Test Plan: unit tests

Reviewers: zasgar, #engineering

Reviewed By: zasgar, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D5704

GitOrigin-RevId: e623f420e4620e5d403122641a7f6e12bf8bdc2a