feat: Add optional json field names map to message type

[GregVS]

Protobuf messages may be stored as JSON objects, for example in a database. Performing query or lookups on these objects often requires a string representation of the message's field names (keys). Currently, the programmer needs to hardcode these strings in their code, but if the underlying protobuf definition changes, there will be no warning that those field names are invalid. Providing an optionally generated mapping of each key to its corresponding JSON name allows Typescript to flag if that field does not exist. This can be enabled using outputJsonFieldNames=true which adds a map object to the message type named $jsonFields.

Example Proto:

message Foo {
  string bar = 1;
}

Typescript Generated Class Member:

const Foo = {
  $jsonFields: {
    bar: "bar"
  } as const,
  ...
}

[stephenh]

@GregVS thanks for the PR! My initial thought is that this seems like a slippery slope to the existing outputTypeRegistry feature.

Granted, I assume you've seen that and don't need everything that it does?

I also get that it's nice to have $jsonFields directly on the Foo const...

I dunno, maybe we could generalize this a little bit and do like:

const Foo = {
  $fields = { bar: { jsonName: "bar" } }
}

With an option like fieldsMetadata=jsonName? Then maybe eventually someone wants fieldsMetadata=jsonName,type and we could do $fields = { bar: { jsonName: "bar", type: "int[]" } } or something like that.

Basically make it a little bit more generic so that $fields could have more things added to it in the future.

Also, if you could include an integration-test/... with a tiny *.proto and then parameters.txt set to use the new flag you're introducing, that would be great as those serve as our regression tests.