[aws-codepipeline-actions] Enable setting filters on github source action

I would like to be able to set the webhook filters on the source action in my code pipeline

### Use Case

I want to trigger the pipeline only when certain files are changed (aka the purpose of the filters)

### Proposed Solution

[Codebuild module](https://github.com/aws/aws-cdk/tree/master/packages/%40aws-cdk/aws-codebuild) seems to support setting webhook filters, but I cannot find a way to do this on the [GitHubSourceAction](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-codepipeline-actions/lib/github/source-action.ts) that I use as the source for my code pipelines. Looking at the code for codepipeline-actions module, it seems like the webhook is set [here](https://github.com/aws/aws-cdk/blob/61865aaef682be6727d7768213260c7a95d799f8/packages/%40aws-cdk/aws-codepipeline-actions/lib/github/source-action.ts#L133). Maybe you could re-use some of the [Codebuild classes](https://github.com/aws/aws-cdk/blob/61865aaef682be6727d7768213260c7a95d799f8/packages/%40aws-cdk/aws-codebuild/lib/source.ts#L192) to implement this for Codepipeline source actions too?

### Other

* [ ] :wave: I may be able to implement this feature request <- but it
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start with packages/@aws-cdk/aws-codepipeline-actions/lib/github/source-action.ts, especially where the webhook is configured, and compare it with webhook filter support in packages/@aws-cdk/aws-codebuild/lib/source.ts. Determine how GitHubSourceAction should expose file-change filters, then verify that a pipeline source action can trigger only for the configured changes.

Written by the indexing model from the issue text.