feat: add --providers to llama stack build (#1718)

# What does this PR do?

allow users to specify only the providers they want in the llama stack
build command. If a user wants a non-interactive build, but doesn't want
to use a template, `--providers` allows someone to specify something
like `--providers inference=remote::ollama` for a distro with JUST
ollama

## Test Plan

`llama stack build --providers inference=remote::ollama --image-type
venv`
<img width="1084" alt="Screenshot 2025-03-20 at 9 34 14 AM"
src="https://github.com/user-attachments/assets/502b5fa2-edab-4267-a595-4f987204a6a9"
/>

`llama stack run --image-type venv
/Users/charliedoern/projects/Documents/llama-stack/venv-run.yaml`
<img width="1149" alt="Screenshot 2025-03-20 at 9 35 19 AM"
src="https://github.com/user-attachments/assets/433765f3-6b7f-4383-9241-dad085b69228"
/>

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>
Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Sébastien Han <seb@redhat.com>

llama_stack/cli/stack/build.py

@@ -75,6 +75,12 @@ the build. If not specified, currently active environment will be used if found.
             default=False,
             help="Run the stack after building using the same image type, name, and other applicable arguments",
         )
+        self.parser.add_argument(
+            "--providers",
+            type=str,
+            default=None,
+            help="Build a config for a list of providers and only those providers. This list is formatted like: api1=provider1,api2=provider2. Where there can be multiple providers per API.",
+        )
 
     def _run_stack_build_command(self, args: argparse.Namespace) -> None:
         # always keep implementation completely silo-ed away from CLI so CLI