feat: allow conditionally enabling providers in run.yaml (#1321)

# What does this PR do?

We want to bundle a bunch of (typically remote) providers in a distro
template and be able to configure them "on the fly" via environment
variables. So far, we have been able to do this with simple env var
replacements. However, sometimes you want to only conditionally enable
providers (because the relevant remote services may not be alive, or
relevant.) This was not possible until now.

To aid this, we add a simple (bash-like) env var replacement
enhancement: `${env.FOO+bar}` evaluates to `bar` if the variable is SET
and evaluates to empty string if it is not. On top of that, we update
our main resolver to ignore any provider whose ID is null.

This allows using the distro like this:

```bash
llama stack run dev --env CHROMADB_URL=http://localhost:6001 --env ENABLE_CHROMADB=1
```

when only Chroma is UP. This disables the other `pgvector` provider in
the run configuration.


## Test Plan

Hard code `chromadb` as the vector io provider inside
`test_vector_io.py` and run:

```bash
LLAMA_STACK_BASE_URL=http://localhost:8321 pytest -s -v tests/client-sdk/vector_io/ --embedding-model all-MiniLM-L6-v2
```

llama_stack/distribution/stack.py

@@ -155,18 +155,34 @@ def replace_env_vars(config: Any, path: str = "") -> Any:
         return result
 
     elif isinstance(config, str):
-        pattern = r"\${env\.([A-Z0-9_]+)(?::([^}]*))?}"
+        # Updated pattern to support both default values (:) and conditional values (+)
+        pattern = r"\${env\.([A-Z0-9_]+)(?:([:\+])([^}]*))?}"
 
         def get_env_var(match):
             env_var = match.group(1)
-            default_val = match.group(2)
+            operator = match.group(2)  # ':' for default, '+' for conditional
+            value_expr = match.group(3)
 
-            value = os.environ.get(env_var)
-            if not value:
-                if default_val is None:
-                    raise EnvVarError(env_var, path)
+            env_value = os.environ.get(env_var)
+
+            if operator == ":":  # Default value syntax: ${env.FOO:default}
+                if not env_value:
+                    if value_expr is None:
+                        raise EnvVarError(env_var, path)
+                    else:
+                        value = value_expr
                 else:
-                    value = default_val
+                    value = env_value
+            elif operator == "+":  # Conditional value syntax: ${env.FOO+value_if_set}
+                if env_value:
+                    value = value_expr
+                else:
+                    # If env var is not set, return empty string for the conditional case
+                    value = ""
+            else:  # No operator case: ${env.FOO}
+                if not env_value:
+                    raise EnvVarError(env_var, path)
+                value = env_value
 
             # expand "~" from the values
             return os.path.expanduser(value)