Alerting: Modify alertmanager endpoints for proxying using the datasource UID

[papagian]

What this PR does / why we need it:
follow-up of #47829
This is part of work of removing the sequential IDs from the API and store layers. For more details you can refer to this doc.

More specifically, it:

• modifies ProxyDatasourceRequestWithUID() so that now optionally expects a string for passing the datasource UID to look for; if it's not provided then the uid path parameter value is used.
• modifies the alertmanager endpoints so that now expect a DatasourceUID path parameter instead of DatasourceID

Example of a successful request to a known datasource UID:

curl -X 'GET' \
  'http://localhost:3000/api/alertmanager/Y4t6ca_Mz/api/v2/silences' \
  -H 'accept: application/json' \
  -H 'authorization: Basic YWRtaW46YWRtaW4='
[{"id":"9b652196-711f-47a9-8032-d7913fdce8a7","status":{"state":"expired"},"updatedAt":"2022-04-15T15:21:40.231Z","comment":"a comment","createdBy":"spapagian","endsAt":"2022-04-15T15:21:40.231Z","matchers":[{"isEqual":true,"isRegex":true,"name":"foo","value":"bar"}],"startsAt":"2022-04-15T15:21:40.231Z"}]

Example of requesting an unknown datasource UID:

curl -X 'GET'   'http://localhost:3000/api/alertmanager/unknown/api/v2/silences'   -H 'accept: application/json'   -H 'authorization: Basic YWRtaW46YWRtaW4='
{"error":"unexpected backend type (unknown)","message":"unexpected backend type (unknown)","traceID":"00000000000000000000000000000000"}

Release notice breaking change

Grafana alerting endpoints prefixed with api/alertmanager/ that proxy requests to an Alertmanager now expect the data source UID as a path parameter instead of the data source numeric identifier.

[joeblubaugh]

If you're up for it, it would be great to make a path function that at least provides the prefix:

AMV2Path(pathSuffix) string {
  return path.Join("/api/alertmanager/datasource/{DatasourceUID}/api/v2", pathSuffix)
}

...

AMV2Path("silence/{SIlenceID")

that makes it easier to scan which API is being evaluated.

If not, I'll create an issue and I can tackle it later.

[yuri-tceretian]

LGTM