feat: add support for running in a venv (#1018)

# What does this PR do?

add --image-type to `llama stack run`. Which takes conda, container or
venv also add start_venv.sh which start the stack using a venv

resolves #1007

## Test Plan

running locally:

`llama stack build --template ollama --image-type venv`
`llama stack run --image-type venv
~/.llama/distributions/ollama/ollama-run.yaml`
...
```
llama stack run --image-type venv ~/.llama/distributions/ollama/ollama-run.yaml
Using run configuration: /Users/charliedoern/.llama/distributions/ollama/ollama-run.yaml
+ python -m llama_stack.distribution.server.server --yaml-config /Users/charliedoern/.llama/distributions/ollama/ollama-run.yaml --port 8321
Using config file: /Users/charliedoern/.llama/distributions/ollama/ollama-run.yaml
Run configuration:
apis:
- agents
- datasetio
...
```

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

llama_stack/cli/stack/run.py

@@ -65,6 +65,13 @@ class StackRun(Subcommand):
             type=str,
             help="Path to TLS certificate file for HTTPS",
         )
+        self.parser.add_argument(
+            "--image-type",
+            type=str,
+            help="Image Type used during the build. This can be either conda or container or venv.",
+            choices=["conda", "container", "venv"],
+            default="conda",
+        )
 
     def _run_stack_run_cmd(self, args: argparse.Namespace) -> None:
         import importlib.resources
@@ -118,11 +125,11 @@ class StackRun(Subcommand):
         config_dict = yaml.safe_load(config_file.read_text())
         config = parse_and_maybe_upgrade_config(config_dict)
 
-        if config.container_image:
+        if args.image_type == ImageType.container.value or config.container_image:
             script = importlib.resources.files("llama_stack") / "distribution/start_container.sh"
             image_name = f"distribution-{template_name}" if template_name else config.container_image
             run_args = [script, image_name]
-        else:
+        elif args.image_type == ImageType.conda.value:
             current_conda_env = os.environ.get("CONDA_DEFAULT_ENV")
             image_name = args.image_name or current_conda_env
             if not image_name:
@@ -167,6 +174,15 @@ class StackRun(Subcommand):
                 script,
                 image_name,
             ]
+        else:
+            # else must be venv since that is the only valid option left.
+            current_venv = os.environ.get("VIRTUAL_ENV")
+            venv = args.image_name or current_venv
+            script = importlib.resources.files("llama_stack") / "distribution/start_venv.sh"
+            run_args = [
+                script,
+                venv,
+            ]
 
         run_args.extend([str(config_file), str(args.port)])
         if args.disable_ipv6: