Accept File Marks for any valid path (via --file-marks-yaml)

[pivotaljohn]

Describe the problem/challenge you have
Tool integrators would like the ability to integrate with ytt, accepting paths to files without limitation.
That integration includes the ability to configure processing of sets of files via File Marks.

Today, file marks are supplied via a command-line flag whose argument is of a specific format that uses the : and = characters as delimiters.
These are valid characters for paths on most operating systems.
This means there are valid path names (however uncommon) for which there is no way to configure a file mark.

Describe the solution you'd like
Introduce a new command-line flag: --file-marks-yaml which contains a sequence of file mark-shaped mappings. In this way, any string can be supplied for path.

For example:

$ ytt -f . --file-marks-yaml '[{"path": "conf:ig.yml", "type": "yaml-plain"}]'

This flag should be able to process all known file marks: https://carvel.dev/ytt/docs/latest/file-marks/#docs

Anything else you would like to add:
This request originated with #468.

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[pivotaljohn]

While this is not on the roadmap for the core maintainers, if someone would fashion this contribution, we'd happily accept. 👍🏻