test: Measure and track code coverage (#2636)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
- Added coverage badge to README. - [See my
fork](https://github.com/ChristianZaccaria/llama-stack)
- Added a GitHub Actions workflow that runs the tests and updates the
coverage badge. - [See
run](4574811323)
- Documented steps in `testing.md` for running the tests locally, and
viewing the `html` report.
- Excluded non-essential files from coverage reporting to provide a more
accurate measurement.

Automatically created PR to update coverage badge:
https://github.com/ChristianZaccaria/llama-stack/pull/9

# Note for reviewers
1. Currently the coverage report shows a 45% coverage. Wondering if
there are other files or directories that should also be excluded from
the report to increase the percentage. The directories with the least
test coverage are `llama_stack/cli`, `llama_stack/models`, and
`llama_stack/ui`. - Should we exclude these?
2. **[Required]** The `GITHUB_TOKEN` should have write permissions to
open a PR to update the coverage badge.

# GitHub Issue
<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #2355 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
The `testing.md` file describes how to run the unit tests locally.

.coveragerc

@@ -4,3 +4,9 @@ omit =
     */llama_stack/providers/*
     */llama_stack/templates/*
     .venv/*
+    */llama_stack/cli/scripts/*
+    */llama_stack/ui/*
+    */llama_stack/distribution/ui/*
+    */llama_stack/strong_typing/*
+    */llama_stack/env.py
+    */__init__.py