Support for taskRunSpecs Property of a PipelineRun with tkn pipeline start

Expected Behavior

Related:
https://github.com/tektoncd/pipeline/issues/2362
https://github.com/tektoncd/pipeline/pull/2389

Pipelines is adding support for allowing TaskRun properties to be defined for TaskRuns that are part of a PipelineRun. As of the current state of https://github.com/tektoncd/pipeline/pull/2389, this will occur via a taskRunSpecs property that is part of a PipelineRun spec.

tkn should support an option to allow a user to specify a taskRunSpec via tkn pipeline start that will allow a user more control over how TaskRuns part of a PipelineRun should be executed.

My initial thought would be to allow a user to define their taskRunSpecs property in a file and then tkn would accept this file via an option:

tkn pipeline start pipelineName --taskrunspec taskrunspec.yaml

tkn pipeline start pipelineName --taskrunspec https://www.github.com/repo/user/taskrunspec.yaml

After parsing the file, tkn would add this property to the PipelineRun.

Open to other options on what the best way to supply this information would be, but, due to how much can be defined for a TaskRun spec, it may be best to allow this information to be stored in file format.

Additional Info

This feature is not currently available in pipelines, so this issue will depend on the final state of #2389 and whenever tkn can update its pipeline dependency with #2389.