Generate client code for expr functions

[imalsogreg]

Functions with expression bodies should result in generated functions usable in client code.

Support:

• Python
• Typescript
• Ruby
• (Stretch) OpenAPI
• (Stretch) Go

We will generate streaming versions iff streaming of return values is available in the runtime. (If streaming return values in the runtime is difficult, it makes sense to release only-nonstreaming client functions, rather than wait).

[linear]

BAML-305 Generate client code for expr functions

[imalsogreg]

Closing and moving to internal tracker