feat: add CODE_FUNCTION_NAME to sentry-conventions, set to route handler function name #23871

Description

Re: #23614 (comment)

@Lms24 suggests:

we might wanna look into if we should add something like a {{code.function.name}} template to the request handler name rules. But if we do, we can tackle it as a follow up

• Update sentry-conventions to make CODE_FUNCTION_NAME a template to optionally hold the request handler function name.
• Update request handlers to follow the convention.