ci: Add a conv. commit lint action+config

[dougch]

Release Summary:

Description of changes:

GitHub Action to check the last commit for compliance with conventional commits.

Call-outs:

I've added a few types from the commit history to the config that aren't common: opt and internal.

Aiming for parity of the config with the https://github.com/aws/s2n-tls project.

Testing:

In this PR's history, you can see a force push that failed when I removed the ci: prefix from the commit.

local:

npm install --save-dev @commitlint/config-conventional @commitlint/cli
cd .github
#Check the last commit
npx commitlint -g ./config/commitlint.config.js --last
# Check between these sha's:
npx commitlint -g ./config/commitlint.config.js --from f51926a63f8b0053186900582e68c916cb4965b5 --to 1e12360a5f9d4045f7beed5cac0e361939c7350e 
⧗   input: Expose TLS error to Subscribers using on_tls_handshake_failed (#2855)
✖   subject may not be empty [subject-empty]
✖   type may not be empty [type-empty]

✖   found 2 problems, 0 warnings
ⓘ   Get help: https://github.com/conventional-changelog/commitlint/#what-is-commitlint

⧗   input: Expose static server name constructor (#2865)
✖   subject may not be empty [subject-empty]
✖   type may not be empty [type-empty]

✖   found 2 problems, 0 warnings
ⓘ   Get help: https://github.com/conventional-changelog/commitlint/#what-is-commitlint

No output means the commit passed, but from the above output you can see that there are a few past commits that would have failed this check.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[boquan-fang]

Should we limit the maximum length for the subject: https://commitlint.js.org/reference/rules.html#subject-max-length? We probably shouldn't allow a PR subject to be larger than one line.

[dougch]

Let's address in a followup, I'm not sure what the right value is here and don't want to guess low.