Add version to REST API url (#478)

# What does this PR do? 

Adds a `/alpha/` prefix to all the REST API urls.

Also makes them all use hyphens instead of underscores as is more
standard practice.

(This is based on feedback from our partners.)

## Test Plan 

The Stack itself does not need updating. However, client SDKs and
documentation will need to be updated.

llama_stack/distribution/stack.py

@@ -40,6 +40,9 @@ from llama_stack.distribution.store.registry import create_dist_registry
 from llama_stack.providers.datatypes import Api
 
 
+LLAMA_STACK_API_VERSION = "alpha"
+
+
 class LlamaStack(
     MemoryBanks,
     Inference,