feat(notification/teams): add teams notification - server side

[sranka]

This PR adds support for Microsoft Teams in InfluxDB notification endpoints and notification rules to the server-side of InfluxDB.
It contains code changes that were reviewed in #18916 (which also contained UI part, which is not subject of this PR)

What is in this PR:

• implementation and tests of teams endpoint and rule
• swagger updated with teams notification rule and endpoint

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Feature flagged (if modified API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[sranka]

If I include swagger update in this PR, UI-dependant automated tests fail (ci/circleci: jlint, jstest, build fail). The root cause is that the UI code cannot be built against types that are generated from an updated swagger file. I could possibly update UI types (as I originally did #18916), but this is AFAIK in a different repo (influxdata/ui). Any advice on how to proceed with this tension between ui and influxdb repositories?

52f99f3 is the swagger change, it is only additive.