Add endpoint test to endpoint configuration

[timhallinflux]

Allow a test message to be sent via provided configuration information. This allows the person configuring the endpoint to verify that the information provided is working.

[russorat]

@desa @nathanielc @kelwang how would something like this work behind the scenes?

[ebb-tide]

Page: http://localhost:8080/orgs/2164f59928cee0a7/alerting/endpoints/:orgID/edit

Should have button that says test

[desa]

Part of this work will be the implementation of the backend for this feature.

[drdelambre]

i lost this ref: #16031