Start PipelineRun from Local or Remote File

[danielhelfand]

Closes #738

This pull request adds the ability to start a PipelineRun from a Pipeline defined in a local or remote file. This follows a similar pattern to tkn task start -f.

It also slightly reorganizes functions in pipeline/start.go so that helper functions are all beneath the actual command function, which is the more common organization throughout the CLI.

Submitter Checklist

• Includes tests (if functionality changed/added)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

Release Notes

Adds the ability to start a PipelineRun from a Pipeline defined in a local or remote file definition

[vdemeester]

/lgtm

[tekton-robot]

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