feat: add rocketchat notifier (prometheus#3600)

* feat: add rocketchat notifier

Signed-off-by: Christoph Maser <[email protected]>
Co-authored-by: George Robinson <[email protected]>

* Update notify/rocketchat/rocketchat.go

Co-authored-by: George Robinson <[email protected]>
Signed-off-by: gotjosh <[email protected]>

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Signed-off-by: Christoph Maser <[email protected]>
Signed-off-by: gotjosh <[email protected]>
Co-authored-by: George Robinson <[email protected]>
Co-authored-by: gotjosh <[email protected]>