[pipelines] Optional CdkPipelineProps.synthAction

Please make the `CdkPipelineProps.synthAction` optional

### Use Case

I am publishing cloud assemblies to S3, and using the S3 source. This means I don't have a need for the synth action, and currently need to hack in an effective no-op action.

### Proposed Solution

`CdkPipelineProps.synthAction?`

### Other

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

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This is a :rocket: Feature Request

Start by locating CdkPipelineProps.synthAction and the pipeline construction that consumes it. Check how an S3 source and cloud assembly are handled, then verify that omitting synthAction works without requiring a no-op action and that the relevant pipeline tests cover this case.

Written by the indexing model from the issue text.