Add Webex incoming webhook alerter

[dennis-trapp]

Description

Adds alerter support for Webex incoming webhook

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[jertel]

Thanks for submitting this. Do you anticipate other webex alerters at some point in the future? I'm asking since you named the new one webex_incoming vs webex.

[dennis-trapp]

There's also a bot functionality for webex https://developer.webex.com/docs/bots and I could see it potentially being added in the future. However I am not too familiar with it as of this moment.

I am open to re-naming the alerter to whatever makes the most sense.

[jertel]

Ok, if there's also a bot option then I agree it's better not to name it just webex. webex_webhook might be more clear but it's not that important.

[dennis-trapp]

webex_webhook makes sense. I'll make that change.