ACA cluster provider support for multiple images + pluggable member store

[sfmskywalker]

Description

This PR refactors the ACA cluster provider with support for multiple Docker images hosted in the same ACA Managed Environment (the cluster).

The way the cluster provider works is as follows:

1. When the provider is constructed, it reads ACA environment variables to get the ACA name, revision and replica name (host name).
2. When the provider is requested to start as a member, it stores the cluster member details using a new abstraction called IClusterMemberStore. The default implementation leverages azure resource tags on the ACA resource, pretty much like what the AmazonECS cluster provider does.
3. The provider starts a background task to periodically poll information from other members in the cluster to update the cluster topology. This, too, relies on the IClusterMemberStore to get all registered members. In the case of the default cluster member store implementation using tags, it basically queries all ACA resources belonging to the same ACA Managed Environment, concatenating all their tags, and constructing them into Member objects.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)