fix(aws-sdk): use custom client_context prop

[smoke]

Which problem is this PR solving?

In every other language the respected property carrying the context is custom e.g.

• https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/9a7f7125ab3f77c47a46be524f29e150536fba6f/instrumentation/aws-sdk/aws-sdk-2.2/library/src/main/java/io/opentelemetry/instrumentation/awssdk/v2_2/internal/LambdaImpl.java#L41

Short description of the changes

• rename Custom to custom property that is used as context carrier for AWS Lambda Client Context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.41%. Comparing base (e713796) to head (db2b6c2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2718   +/-   ##
=======================================
  Coverage   92.41%   92.41%           
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  Files         171      171           
  Lines        8150     8150           
  Branches     1653     1653           
=======================================
  Hits         7532     7532           
  Misses        618      618           

Files with missing lines	Coverage Δ
...try-instrumentation-aws-sdk/src/services/lambda.ts	97.77% <ø> (ø)