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

**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](https://carvel.dev/ytt/docs/latest/file-marks/#docs).

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:
```bash
$ 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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No files or tests are named. Start by locating the existing file-marks command-line flag parsing and the code that converts marks into processing behavior. Trace how each known file-mark type is handled, then verify that the YAML flag accepts arbitrary paths and supports all known marks with focused tests.

Written by the indexing model from the issue text.