[Mattermost] Lax requiring recipients and set raw recipients on cfg init

[kimlisa]

While testing the standalone mattermost plugin from: gravitational/teleport-plugins#876, ran into an error where raw recipients (from config) were not getting notified b/c the raw recipients were not being set on config init which this PR fixes and tests.

I also defined toml fields. It wasn't necessary but figured it wouldn't hurt to if the go struct field name changes for whatever reason

Also laxes requiring team/channel, this was the original implementation (something I failed to understand when refactoring). It's okay for these fields to be left blank, because the mattermost plugin also sends notifications to the suggested reviewers defined in the access request (as long as it can be matched with mattermost users email).

If there are no team/channel, email, or suggested reviewers defined then the plugin logs this:

WARN   No channel to post request_id:7a440f0a-df3b-4bed-95f6-a632e5afa19a request_op:put request_state:PENDING common/app.go:258

if there is a suggested reviewer that doesn't exist in mattermost, the plugin logs this:

WARN   Failed to notify a suggested reviewer: "random-user-1" does not look like a valid email request_id:48eb57d0-8164-42cf-81bc-d341d6174664 request_op:put request_state:PENDING common/app.go:406

[public-teleport-github-review-bot]

@kimlisa See the table below for backport results.

Branch	Result
branch/v13	Create PR