Alerting: Convert receiver testing API to app-platform SDK

This is part of migrating to Grafana app-platform SDK. ### Background The current model of API is unnecessary complex and rigid for the use-case: - it allows testing many integrations in many receivers. The UI never used this capabilities and allows to test only one integration at a time. There are no plans to change this in the future. - the permission model is too basic - the API is protected with one non-scoped permission and ignores access to receivers while accessing the existing ones to patch the secrets. ### Proposal 1. Create an API endpoint, a sub-resource for the receiver resource, i.e. `.../receivers/$UID/test` a. The payload can contain either just UID of integration to test or full definition of an integration (no integration uid is required) or partial definition of existing integration that will be patched with secrets. 2. To cover one exceptional use-case, when a user creates a new receiver and wants to test it, the API will have a placeholder for receiver UID, an impossible UID for receivers, e.g. "-". 3. The test permission will become scoped by UID of receiver. a. Current non-scoped permissions are migrated to wildcard b. Access to receiver under test is required: - for non-modified test (just receiver and integration UID are provided) API will require `read + test` - for modifying tests (receiver UID and full or partial integration is provided) - `read + write + test` (+ protected in case there are changes of protected fields) - for new receiver (no receiver uid) - `create` only (TBD) (alternatives: a new permission e.g. "test-new" or statically scoped test in "-" scope) The result of the test will be: - status (success or failure) - duration of test - error message