In order for a CodeBuild to run in batch mode, a batch service role is needed, as described [here in the docs](https://docs.aws.amazon.com/codebuild/latest/userguide/batch-build.html).

>Batch builds introduce a new security role in the batch configuration. This new role is required as CodeBuild must be able to call the StartBuild, StopBuild, and RetryBuild actions on your behalf to run builds as part of a batch. Customers should use a new role, and not the same role they use in their build...

At first I thought lets add this by default, but then I realised when `BatchConfiguration` is set to something, in the aws console the default 'start build' button behaviour changes to start a batch build by default instead :/

So now this adds a new `supportBatchBuildType` option, which when `true` adds minimum amount of `BatchConfiguration` needed for batch builds to run.

I also updated the doc blocks for the webhook option and CodePipeline action option, because users of those also need to set this option. It would be nice to auto-enable this if a webhook or CodeBuild action is configured, but that sounds pretty complicated.

I'm not sure why anyone would need to customise this role, given it appears to only be used internally to do those 3 things, so this PR does not make it configurable. My thinking is that this could be added later if needed, but this PR just gets batch builds working.

In the future if people want control of the other `BatchConfiguration` options I was thinking these could be added and would require `supportBatchBuildType` to be `true`.

related: aws-cloudformation/cloudformation-coverage-roadmap#621

Closes #12175

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Manually resolved files:
- packages/@aws-cdk/aws-codebuild/lib/project.ts
- packages/@aws-cdk/aws-codebuild/lib/source.ts

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

See [CHANGELOG](https://github.com/aws/aws-cdk/blob/bump/2.0.0-alpha.3/CHANGELOG.md)