add support for additional files

[jaimergp]

This PR adds a new key, extra_files. From the help message:

Extra, non-packaged files that should be added to the installer. If provided as relative
paths, they will be considered relative to the directory where construct.yaml is.
This setting can be passed as a list of:

• str: each found file will be copied to the root prefix
• Mapping[str, str]: map of path in disk to path in prefix.

A new example, extra_files has been included.

Closes #465

[hoechenberger]

Great work! Any chance we could see this getting merged sometime soon?

[larsoner]

Any chance to at least test the Linux case in tests/test_header.py ?

Seeing these PRs and functionality for each OS makes me think we could really use tests for more than just Linux for all these options

[jaimergp]

@larsoner This PR includes an example YAML using these keys, which means the CI will build and run the installers. The only thing we are not testing is whether the installation contains those files.

[jaimergp]

I finally figured out how to use post_install scripts to run some tests. The PR now includes tests for this feature, and also adds some for the branding PR (custom location stuff). I'll add more post-install tests in a new PR to make sure everything is tidy and clean!

@larsoner @hoechenberger can you take another look? Thanks!