Consider additional flags for code generation

### Background

Currently, the _generate_ command has the following subcommands that are available for use with code generation:

- _all_: Generate code for provider, resources, and data sources from an Intermediate Representation (IR) JSON file.
- _data-sources_: Generate code for data sources from an Intermediate Representation (IR) JSON file.
- _provider_: Generate code for the provider from an Intermediate Representation (IR) JSON file.
- _resources_: Generate code for resources from an Intermediate Representation (IR) JSON file.

### Proposal

Consider the introduction of flags which would allow defining for which data source(s) and resource(s) code should be generated.

Consider the introduction of flags which would allow specifying whether code should be generated for _schema_, _model(s)_, _model helper functions_, and/or __expand_/flatten_ functions.