(feature, csharp): Add customizable exception class names

[amckinney]

This adds the ability to customize the base Exception and ApiException class names generated in the SDK. This is controlled via the following:

- name: fernapi/fern-csharp-sdk
  version: 0.9.0
  config:
    base-api-exception-class-name: AcmeApiException
    base-exception-class-name: AcmeException

Note that this also introduces a thin validation layer when we parse the custom configuration, which prevents users from specifying the same configuration value for both classes (which would inevitably create a conflict).