Allow for file exclusions based on entries in an .imgpkgignore file - similar to .gitignore

**Describe the problem/challenge you have**
[A description of the current challenge that you are experiencing.]

I'm creating an image for the contents of a git repo, would like to exclude some files/directories when the image is generated - I can try to use --file-exclusion but it would be convenient to have something like a `.imgpkgignore` file, similar to `.gitignore` or `.tiltignore`

**Describe the solution you'd like**
[A clear and concise description of what you want to happen. If applicable a visual representation of the UX (ex: new CLI argument name, the behavior expected).]

it would be convenient to have something like a `.imgpkgignore` file, similar to `.gitignore` or `.tiltignore` where I could specify these file exclusions

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

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Start by locating the CLI implementation of --file-exclusion and the image-generation entry point. Compare the requested .imgpkgignore behavior with the existing exclusion mechanism and the referenced .gitignore or .tiltignore patterns. Done means exclusions declared in .imgpkgignore are applied when the image is generated.

Written by the indexing model from the issue text.