[SLS-1361] Use DD_XXX env vars for tagging

[lizapressman]

What does this PR do?

Allows for user to set parameters as:

These get translated to DD_XXX environment variables if using the extension and added as tags if using the forwarder.

If DD_XXX or tag is specifically set at the function level, that env var or tag will not be overwritten.

Motivation

https://datadoghq.atlassian.net/browse/SLS-1361

Testing Guidelines

tested with old and new unit tests

Additional Notes

Types of changes

• [ ] Bug fix
• [x] New feature
• [x] Breaking change ((For versions 9 or lower of the extension))
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [x] This PR contains breaking changes that are documented in the description
• [ ] 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 #63 (ca40784) into master (46d5a8d) will increase coverage by 2.45%. The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   90.45%   92.91%   +2.45%     
==========================================
  Files           7        7              
  Lines         482      494      +12     
  Branches      111      116       +5     
==========================================
+ Hits          436      459      +23     
- Misses         22       26       +4     
+ Partials       24        9      -15     

Impacted Files	Coverage Δ
serverless/src/tags.ts	94.73% <91.30%> (-5.27%)	:arrow_down:
serverless/src/env.ts	96.70% <100.00%> (+3.11%)	:arrow_up:
serverless/src/index.ts	91.42% <100.00%> (+1.42%)	:arrow_up:
serverless/src/redirect.ts	82.60% <0.00%> (-0.73%)	:arrow_down:
serverless/src/layer.ts	98.85% <0.00%> (+2.29%)	:arrow_up:
serverless/src/tracing.ts	80.59% <0.00%> (+2.98%)	:arrow_up:
serverless/src/forwarder.ts	94.91% <0.00%> (+5.08%)	:arrow_up:

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