feat: remove usage of build yaml (#4192)

# What does this PR do?

the build.yaml is only used in the following ways:

1. list-deps
2. distribution code-gen

since `llama stack build` no longer exists, I found myself asking "why
do we need two different files for list-deps and run"?

Removing the BuildConfig and altering the usage of the
DistributionTemplate in llama stack list-deps is the first step in
removing the build yaml entirely.

Removing the BuildConfig and build.yaml cuts the files users need to
maintain in half, and allows us to focus on the stability of _just_ the
run.yaml

This PR removes the build.yaml, BuildConfig datatype, and its usage
throughout the codebase. Users are now expected to point to run.yaml
files when running list-deps, and our codebase automatically uses these
types now for things like `get_provider_registry`.

**Additionally, two renames: `StackRunConfig` -> `StackConfig` and
`run.yaml` -> `config.yaml`.**

The build.yaml made sense for when we were managing the build process
for the user and actually _producing_ a run.yaml _from_ the build.yaml,
but now that we are simply just getting the provider registry and
listing the deps, switching to config.yaml simplifies the scope here
greatly.

## Test Plan

existing list-deps usage should work in the tests.

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>

tests/external/ramalama-stack/build.yaml

@@ -1,13 +0,0 @@
-version: 2
-distribution_spec:
-  description: Use (an external) Ramalama server for running LLM inference
-  container_image: null
-  providers:
-    inference:
-    - provider_type: remote::ramalama
-      module: ramalama_stack==0.3.0a0
-image_type: venv
-image_name: ramalama-stack-test
-additional_pip_packages:
-- aiosqlite
-- sqlalchemy[asyncio]