Make sure a version is defined and defaulted for client options

One lint we should write for #3404 is because of a change we are making for #3368 (or related): because we'll add an emitter option to omit emitting of constructors and client options, we need to make sure that an `api_version` or similarly authored[^tsp] version is not only defined but defaulted per [guidelines](https://azure.github.io/azure-sdk/rust_introduction.html#rust-client-configuration-default).

[^tsp]: This is defined by TypeSpec, but I'm not sure if we can access that at linting time.

Start by reviewing issues #3404 and #3368, then inspect the emitter's linting entry points and the linked Rust client-configuration guideline. Determine whether the TypeSpec-defined version is available during linting; done means a lint verifies that the version is defined and defaulted according to the guideline.

Written by the indexing model from the issue text.