Add MSTeams CodeBlock element

[MichaelUrman]

This adds support for the extension element CodeBlock.

[atc0005]

@MichaelUrman

Thanks again for submitting this PR and the discussion on GH-288.

I'll rebase this branch and then merge it.

As noted on GH-288 I'll submit a follow-up PR for the other items.

[atc0005]

CI failures:

• typo (will fix later)
• linting warning regarding a feature added in Go 1.16, deprecated in Go 1.19
  • this project's go.mod file specifies Go 1.14 (it doesn't currently use any features from a newer Go version), so this linting warning should not be surfaced
  • considering this an issue with the CI configuration instead of this project

EDIT:

Submitted upstream bug report for item 2:

• staticcheck linter does not exclude checks for Go versions greater than specified in go.mod file golangci/golangci-lint#4948