Adds option to use the GovCloud lambda layer

[DylanLovesCoffee]

What does this PR do?

Adds the parameter of isGovCloud to have the GovCloud lambda layer be added to the user's functions.

Motivation

SLS-701 - the GovCloud layer has a different ARN to be accounted for.

Testing Guidelines

Added new unit tests to cover this case.

Additional Notes

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] 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
• [ ] 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)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 1.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   87.64%   88.69%   +1.04%     
==========================================
  Files           7        7              
  Lines         340      345       +5     
  Branches       79       81       +2     
==========================================
+ Hits          298      306       +8     
+ Misses         21       20       -1     
+ Partials       21       19       -2     

Impacted Files	Coverage Δ
serverless/src/env.ts	94.44% <ø> (ø)
serverless/src/index.ts	90.19% <100.00%> (ø)
serverless/src/layer.ts	98.07% <100.00%> (+6.58%)	:arrow_up:

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