[serverless] Tag lambdas provisioned with CDK

[Czechh]

What does this PR do?

Adds a tag lambda:createdBy with the value CDK for lambdas that are provisioned with the AWS CDK. It follows the same tag key as SAM

Motivation

Serverless telemetry

Testing Guidelines

• Manually tested
• Unit tested

Additional Notes

Types of changes

• [ ] Bug fix
• [ ] 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
• [ ] 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

[tianchu]

I'm a little bit concerned that this may mislead people to think this tag is created by AWS, also wondering if this would cause some conflict in the future if AWS does decide to tag something like this. Perhaps we should let macro add these two tags for consistency dd_sls_macro_by:sam (despite the fact that AWS already add a tag for SAM created function) and dd_sls_macro_by:cdk?

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 0.50%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   88.66%   89.16%   +0.50%     
==========================================
  Files           7        7              
  Lines         344      360      +16     
  Branches       81       85       +4     
==========================================
+ Hits          305      321      +16     
  Misses         20       20              
  Partials       19       19              

Impacted Files	Coverage Δ
serverless/src/index.ts	90.74% <100.00%> (+0.54%)	:arrow_up:
serverless/src/tags.ts	100.00% <100.00%> (ø)

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[Czechh]

@tianchu pushed the change for adding a brand new tag:

Testing with SAM: )

Testing with CDK:

Please re-review