Add Prometheus AlertManager alert provider with full test coverage

[bosmanx]

Summary

This PR adds comprehensive test coverage to complete PR #515 by @varuzam.

The original PR #515 implements the AlertManager alert provider with support for:

• Single and multiple AlertManager endpoints
• Environment variable and parameter-based configuration
• Basic authentication
• Alert sending with configurable duration

This PR adds the missing unit tests (16 comprehensive tests) to allow the feature to be merged.

Minor fix: endpoints parameter was incorrectly required in __init__ - now properly optional when using environment variables.

Pull Request Checklist

• All pull requests should be against the develop branch, not master.
• All pull requests must include the Contributor License Agreement (see below).
• Whether or not your PR includes unit tests:
  • Please make sure you have run the exact code contained in the PR locally, and it functions as desired.
  • Please make sure the TravisCI build passes or, if not, you've corrected any obvious problems identified by the tests.
• Code should conform to the Development Guidelines:
  • pep8 compliant with some exceptions (see pytest.ini)
  • 100% test coverage with pytest (with valid tests) - 16 tests covering all AlertManager functionality
  • Complete, correctly-formatted documentation for all classes, functions and methods.
  • documentation has been rebuilt with tox -e docs (requires Python 3.9, will be handled by CI)
  • Connections to the AWS services - N/A (AlertManager is an alert provider, not AWS service)
  • All modules should have (and use) module-level loggers.
  • Commit messages should be meaningful, and reference the Issue number
  • Git history is fully intact; please do not squash or rewrite history.

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