Add support for Secrets Manager

[duncanista]

What does this PR do?

Add support for AWS Secrets Manager for rotating API keys.

Motivation

Refer to SLS-1651.

Testing Guidelines

• Adding test cases to check multiple api keys.
• Not add the key in node runtimes where synchronous metrics is being used, since it's not supported.
• Correctly add the key for every other case.

Additional Notes

N/A

Types of changes

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

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] 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
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #52 (4a17153) into master (083f1ca) will increase coverage by 0.44%. The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   89.95%   90.39%   +0.44%     
==========================================
  Files           7        7              
  Lines         458      479      +21     
  Branches      100      108       +8     
==========================================
+ Hits          412      433      +21     
  Misses         22       22              
  Partials       24       24              

Impacted Files	Coverage Δ
serverless/src/env.ts	93.58% <95.65%> (+2.36%)	:arrow_up:
serverless/src/layer.ts	96.42% <100.00%> (ø)

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