Add ability to define comment using static text

[skennedy]

Was wondering if you would accept a PR to allow specifying a PR comment using static text, rather than file contents?

This is very much inspired by the slack resource (https://github.com/cloudfoundry-community/slack-notification-resource#parameters), where you can define your message using static text which can be parameterised by the build metadata.

Without this feature, it is surprisingly tricky to write comments incorporating this metadata - because metadata is hidden from task steps by design - so I had to resort to using swce/metadata-resource and then define a task to create the comment file using the metadata. All rather cumbersome.

[crstamps2]

Yeah, I think this is a great idea. I would accept a PR on it.