Add Source Code Integration param and SAM usage instructions

[sfirrin]

What does this PR do?

Adds a param to the macro that can be used with SAM's --parameter-overrides option and local git commands to automatically add the git.commit.sha and git.repository_url tags used by Datadog Source Code Integration.

Motivation

Easy set up and usage of SCI

Testing Guidelines

Deployed with a sample SAM app with and without setting the param.

Additional Notes

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [x] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [x] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #82 (e967750) into master (0586cf2) will decrease coverage by 1.11%. The diff coverage is 66.66%.

:exclamation: Current head e967750 differs from pull request most recent head 32bcb83. Consider uploading reports for the commit 32bcb83 to get more accurate results

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   88.49%   87.39%   -1.11%     
==========================================
  Files           7        8       +1     
  Lines         565      595      +30     
  Branches      145      151       +6     
==========================================
+ Hits          500      520      +20     
- Misses         55       64       +9     
- Partials       10       11       +1     

Impacted Files	Coverage Δ
serverless/src/env.ts	75.92% <14.28%> (-2.79%)	:arrow_down:
serverless/src/git.ts	82.60% <82.60%> (ø)

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more