feat: Add support for starting position AT_TIMESTAMP to MSK events

[griffithsbs]

Closes: #12033

• Add support for a StartingPosition of AT_TIMESTAMP to MSK event source mappings. When this configuration is used,
  StartingPositionTimestamp must also be provided, in Unix Time seconds, which is consistent with the existing support in the kafka event

• Support for this starting position has recently been added to AWS Lambda event source mappings using MSK
  (https://aws.amazon.com/about-aws/whats-new/2023/06/aws-lambda-starting-timestamp-kafka-sources/)

• Since runServerless tests are expensive, test the happy path in the existing test of a valid configuration rather than adding an explicit separate test for it, but add a new test to explicitly verify the unhappy path in which a timestamp at which to start is not provided

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0382fac) 86.70% compared to head (bf564d0) 86.71%.

❗ Current head bf564d0 differs from pull request most recent head d8a3722. Consider uploading reports for the commit d8a3722 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12034   +/-   ##
=======================================
  Coverage   86.70%   86.71%           
=======================================
  Files         316      316           
  Lines       13513    13520    +7     
=======================================
+ Hits        11717    11724    +7     
  Misses       1796     1796           

Impacted Files	Coverage Δ
...ib/plugins/aws/package/compile/events/msk/index.js	100.00% <100.00%> (ø)
...ckage/compile/events/websockets/lib/authorizers.js	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[medikoo]

Looks great 👍

[kaykhan]

@medikoo @griffithsbs I noticed in serverless.ts AT_TIMESTAMP is still not an option, note StartingPositionTimestamp is also not available. https://docs.aws.amazon.com/lambda/latest/dg/with-msk.html

[medikoo]

@kaykhan serverless.ts is maintained by external contributors at https://github.com/serverless/typescript

[kaykhan]

@kaykhan serverless.ts is maintained by external contributors at https://github.com/serverless/typescript

Oh i did not know that, that makes sense why some things are out of sync sometimes, thanks!