Namespace refactoring Implementation

[lmazuel]

No description provided.

[weidongxu-microsoft]

epic

• Support multiple namespaces in client library code generation microsoft/typespec#4537

design

• https://gist.github.com/srnagar/a695faa1a4d8a5dbf44af826f05916f0

depends

• [tcgc] namespaces in TCGC typespec-azure#1604
• [tcgc] provide a name for SDK client (when SDK client is from TypeSpec Namespace) typespec-azure#1737
• fix in typespec about the namespace of template instance (it should be of the namespace of where instantiation take place)