Directly generate ToSchema instance from .proto AST

[Gabriella439]

The motivation for this change is to fix the mismatch between the Swagger API
declared in ToSchema instances and the actual API served by the FromJSONPB
and ToJSONPB instances.

The difference between the two was in casing conventions: the JSONPB API was
preserving the original case of the fields from the .proto API, whereas the
Swagger API was camel-casing the declared fields.

The reason they differ was that they were going through two separate code
pathways. In the case of JSONPB the FromJSONPB and ToJSONPB instances were
being generated directly from the .proto AST without any intermediate steps:

`.proto` AST →(code generation)→ `FromJSONPB`

`.proto` AST →(code generation)→ `ToJSONPB`

... and that code generation step preserved the original case.

However, the ToSchema instance was generated indirectly via GHC generics:

`.proto` AST →(code generation)→ Haskell data type →(Generics)→ `ToSchema`

... and the code generation step for the Haskell data type was not preserving
case.

After this change the code generation directly generates the ToSchema
instance:

`.proto` AST →(code generation)→ `ToSchema`

Some alternative approaches that I considered and rejected:

• Change the Haskell data type generation to preserve the original case instead
  of camel-casing Haskell field names

  I rejected this approach because it is highly disruptive to downstream code
  and also generates ugly field names

• Customize the ToSchema instances derived via Generics to convert camel
  case to snake case

  I rejected this approach because it is not correct if the original .proto
  field names were already camel-cased. This would then lead to the opposite
  problem: declaring a snake-cased field which was actually camel-cased

I tested this downstream on our large internal .proto files. The only
difference in the generated Swagger is that the casing for declared fields is
fixed and the generated Haskell code still compiles without any changes to the
client packages.

I also added some simple golden tests for simple visual inspection of what the
generated Swagger looks like.

[intractable]

I didn't do deep review, but this LGTM and it's already been tested. Thank you so much for doing this!