[TCGC] support an option to "keepAll" models

Currently .Net generator has an option controlling what we should do for those unreferenced types. One of the options is `keepAll` which allows the user to get all the models they defined in their spec.
But those unreferenced types would not have usage, therefore currently TCGC will remove them.
Despite users could use `@@usage` to explicitly claim they want to keep this, but this is very hard to use when they have a lot of models. This happens in openai spec.

Start by tracing how TCGC removes unreferenced types and how the .Net generator handles the `keepAll` option. Check the existing generator tests for coverage of unreferenced models. Done means `keepAll` preserves every model defined in the spec without requiring individual `@@usage` declarations, with regression coverage.

Written by the indexing model from the issue text.