Authentication for /alive endpoint

[przemeklal]

Enhancement Proposal

Currently:

• client ID (potentially sensitive) is sent over the network, without native TLS support in cos-alerter this means sending sensitive data unencrypted
• even with HTTPS enabled, a malicious actor can send a valid client ID to the /alive endpoint and pretend to be an active alertmanager instance while the actual instance may be down

Potential solutions:

• TLS support + a pre-shared secret (password) included in the client POST requests sent to /alive (ins tead of sending only client ID that may be easy to guess)
• sending a token in the POST request and mapping that token to a client ID in cos-alerter.yaml config
• mutual TLS (might be an overkill)

[lucabello]

We've implemented authentication using a client ID. Without TLS however this isn't secure; once TLS is implemented this can be closed.

[lucabello]

Closing this in favor of #59, so we can track the progress in a single place.