feat(api): add receiver labels and receiver_matchers filter (#5152)

Add support for arbitrary key-value labels on receivers, enabling
structured identification for querying and filtering. The receiver
name is auto-injected as the "name" label for uniform matcher access.

Expose labels in all API receiver responses (GET /receivers,
GET /alerts, GET /alerts/groups) and add a receiver_matchers query
parameter to all three endpoints for filtering by receiver labels
using PromQL-style matchers.

Ref: #4963

Signed-off-by: Cody Kaczynski <ckaczyns@akamai.com>
Signed-off-by: Solomon Jacobs <solomonjacobs@protonmail.com>
Co-authored-by: Solomon Jacobs <solomonjacobs@protonmail.com>