content.outcome field is not declared as enums

[e-backmark-ericsson]

The documentation for pipelineRun.finished and taskRun.finished declares that the outcome shall be an enum with a limited set of values. The schemas declare these fields as free text strings though.

We need to align this, and at the same time I believe we should decide on and document how we should format these kinds of enum values. I prefer to see them as "constants" and use the well established formatting of UPPERCASE values for them, but I'm open for other decisions as well, as long as we align our enum values' formats all-over the protocol

[e-backmark-ericsson]

Partly covered in #158 and fully covered in #179