Azure mysql postgres auto discovery configuration

[GavinFrazar]

This PR adds configuration for Azure database auto-discovery.

This is the first of several PRs to implement #14688

Configuration allows a user to filter by subscription, resource group, region, tags. If subscription or resource group is omitted, then all subscriptions or all resource_groups are queried. Users can also use * to specify this behavior explicitly. Example configs:

db_service:
  enabled: "yes"
  resources:
    - labels:
        "*": "*"
  azure:
    # discover all postgres DBs in all subscriptions and resource groups, in the eastus region.
    - subscriptions: ["*"]
      resource_groups: ["*"]
      types: ["postgres"]
      regions: ["eastus"]
      tags:
        "*": "*"
    # discover all mysql DBs in all subscriptions and resource groups, in the eastus or centralus.
    - types: ["mysql"]
      regions: ["eastus", "centralus"]
      tags:
        "*": "*"
    # discover all mysql and postgres DBs a specific subscription and resource group, in the eastus region
    - subscriptions: ["060a97ea-3a57-4218-9be5-dba3f19ff2b5"]
      resource_groups: ["database-access"]
      types: ["mysql", "postgres"]
      regions: ["eastus"]
      tags:
        "*": "*"

[GavinFrazar]

Force pushed to split out some unneeded code and restructure my commits.

[Tener]

As far as the configuration goes, apart from the small comments, I think this looks good.

It would be good to see the larger context still, i.e., actual db watcher code that would consume this configuration and execute the matching logic itself.

[codingllama]

Just dropping by to say that the PR sizes are excellent now, thanks for the refactors @GavinFrazar. 🤌

You've got a healthy number of active reviewers, so I'll leave it to them.

[GavinFrazar]

@smallinsky friendly ping for re-review, since I need approval from both reviewers list. I think I addressed all of your concerns.