Separate job configurations for synth/publish/default

[nick-kang]

I'd like to suggest adding the ability to provide job configurations to synth, publish, and default stages. This would be similar to the synthCodeBuildDefaults, codeBuildDefaults, assetPublishingCodeBuildDefaults properties in pipelines.CodePipeline(https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.pipelines.CodePipeline.html#synthcodebuilddefaults).

The use case is that my synth job is compute intensive, whereas my asset publishing job is just uploading artifacts. I'd like to configure the synth job to use a more powerful self-hosted runner instance, but don't want to overprovision for the asset publishing step.