Support UTF-8 format for Cortex Alert manager label matchers

[rakheshkumbi]

Is your feature request related to a problem? Please describe.
For the "Add notification policy" page, it will always report error validating Alertmanager config: bad matcher format: "a"="b" when I try to any matching labels (I am using cortex alertmanager here), but it works if use the REST API of cortex.

cortex REST API:
curl -i -H "Authorization: Bearer $(TOKEN)" -H 'Content-Type: application/yaml' -X POST --data-binary @alerts.yaml $(ALERT_URL)/api/v2/alerts
alerts.yaml :

alertmanager_config: |

route:
- continue: false
matchers:
- a=b
mute_time_intervals: []
receiver: simin_contact_point
routes: []

Describe the solution you'd like
Currently the cortex doesn't support UTF-8 format for label matchers. If cortex can support the UTF-8 format for the label matcher, the issue can be resolved.

Describe alternatives you've considered
NA

Additional context
It should be possible to add any existing or not-existing matching labels in Add notification policy" page of grafana without error