Vendor the load function from the Lambda runtime

[nhinsch]

What does this PR do?

Copy the load function (and its supporting code) from the Lambda runtime directly into our library.

Motivation

We rely on the load function to load the customer's Lambda function from within our wrapper function. Unfortunately it's not safe to import this function directly from the Lambda runtime, as it is not necessarily stable. In order to avoid introducing a breaking change and to ensure all edge cases are handled correctly, I copied the code directly from the runtime.

Testing Guidelines

Integration tests show that the code is still loaded correctly.

Types of Changes

• Bug fix
• 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
• This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

Merging #238 (73d6ddc) into main (4738b4a) will decrease coverage by 5.33%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
- Coverage   86.91%   81.57%   -5.34%     
==========================================
  Files          32       35       +3     
  Lines        1253     1335      +82     
  Branches      266      283      +17     
==========================================
  Hits         1089     1089              
- Misses         96      178      +82     
  Partials       68       68              

Impacted Files	Coverage Δ
src/handler.ts	0.00% <0.00%> (ø)
src/runtime/errors.ts	0.00% <0.00%> (ø)
src/runtime/index.ts	0.00% <0.00%> (ø)
src/runtime/user-function.ts	0.00% <0.00%> (ø)

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[DarcyRaynerDD]

LGTM, but let's make sure the AWS runtime repo is in the 3rd-party license file as well.