add support to filter which files from an azd template are copied during init

In the ai agent extension, we rely on https://github.com/Azure-Samples/azd-ai-starter-basic.

This can work OK if you start from a totally empty repo directory because azd init -t is going to copy _everything_ from the repo (shallow clone). For existing repos / projects, and running `azd ai agent init`, we don't want all the other repo parts of the template (README, .gitignore, issue templates, etc.). We only need the azure.yaml and infra/ directory.

We need a way to filter what is used from a template, akin to .azdignore that our tools can use.

Here is an example of everything I needed to remove from the official template for the ai agent scenario: https://github.com/Azure-Samples/azd-ai-starter-basic/compare/main...therealjohn:azd-ai-starter-basic:main