Support exact() Function for @clientName (TCGC 0.68.0)

[lirenhe]

Docs https://azure.github.io/typespec-azure/docs/howtos/generate-client-libraries/09renaming/#preserving-exact-casing
A new exact() function that preserves client names without language-specific casing transformations. Previously, names provided via @clientName would still be subject to emitter-specific casing rules (e.g., PascalCase for C#, snake_case for Python). Now you can opt out of those transformations entirely.

Java emitter should support this function.