Alerting: Introduce a Mimir client as part of the Remote Alertmanager (#78357)

* Alerting: Introduce a Mimir client as part of the Remote Alertmanager

This is our first attempt at making Grafana communicate use Mimir as a backend - it uses a new set of APIs that we've developed on the Mimir side to upload the grafana configuration and alertmanager state so that it can then be ported over.

Codewise, we've introduced a couple of things:

A client to isolate in its own package all the communication that happens with Mimir
A few changes to the remote/alertmanager to include uploading the configuration and state when it starts
A few refactors that align a bit better with the design approach that we're thinking
An integration tests again these newly developed APIs using a custom image

---------

Signed-off-by: gotjosh <josue.abreu@gmail.com>
Co-authored-by: Santiago <santiagohernandez.1997@gmail.com>

scripts/drone/steps/lib.star

@@ -971,7 +971,6 @@ def remote_alertmanager_integration_tests_steps():
 
     environment = {
         "AM_TENANT_ID": "test",
-        "AM_PASSWORD": "test",
         "AM_URL": "http://mimir_backend:8080",
     }
 

scripts/drone/utils/images.star

@@ -20,7 +20,7 @@ images = {
     "plugins_slack": "plugins/slack",
     "python": "python:3.8",
     "postgres_alpine": "postgres:12.3-alpine",
-    "mimir": "grafana/mimir:latest",
+    "mimir": "us.gcr.io/kubernetes-dev/mimir:gotjosh-state-config-grafana-663a0ae78",
     "mysql5": "mysql:5.7.39",
     "mysql8": "mysql:8.0.32",
     "redis_alpine": "redis:6.2.11-alpine",