allow customization of the yaml template

[emcake]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Add an extra configurable YAML_TEMPLATE argument to the KubernetesAgent to allow overwriting of the template job spec.

Why is this PR important?

Due to environmental setup of our k8s cluster I found myself needing to alter the podspec run in ways not exposed by current environment variables, and it didn't seem sensible to add new environment variables for every change I'd want to make to the template. This approach allows me to build a custom container with a specialized yaml spec.

[marvin-robot]

Here I am, brain the size of a planet and they ask me to welcome you to Prefect.

So, welcome to the community @emcake! 🎉 🎉

[codecov]

Codecov Report

Merging #3046 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

[joshmeek]

Hi @emcake it looks like your local git may not be authenticated with your GitHub account! Generally you can see this when your commits show up as the grey octocat logo.

If you want to resolve this (and attribute the commits to yourself) you could try options like this:

• https://docs.github.com/en/enterprise/2.18/user/github/committing-changes-to-your-project/why-are-my-commits-linked-to-the-wrong-user#commits-are-not-linked-to-any-user
• https://docs.github.com/en/github/setting-up-and-managing-your-github-user-account/setting-your-commit-email-address#setting-your-commit-email-address-in-git

Other than that could you also run black . over your branch for formatting and add this enhancement to the /changes directory 🙂