chore: statically extract .proto from Go source

[alecthomas]

This adds a general purpose command go2proto that statically generates a .proto file from Go source code. This used to be done via reflection, but that resulted in a chicken and egg problem whereby the reflection code needed to be compilable in order to generate the protobuf files that the reflection code depended on.

A couple of minor side-effects:

1. This will automatically extract sum types and enums.
2. The runtime types have been moved directly into the schema, as opposed to only existing in the proto.
3. Sum type elements will need to be tagged with a //protobuf:<id> directive.

These are the breaking changes:

backend/protos/xyz/block/ftl/v1/schema/schema.proto:43:26:Field "5" on message "Data" changed name from "typeParameters" to "type_parameters".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:162:3:Field "2" with name "kind" on message "MetadataAlias" changed type from "int64" to "enum".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:197:10:Field "3" on message "MetadataRetry" changed name from "minBackoff" to "min_backoff".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:198:10:Field "4" on message "MetadataRetry" changed name from "maxBackoff" to "max_backoff".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:210:10:Field "3" on message "MetadataTypeMap" changed name from "nativeName" to "native_name".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:245:17:Field "4" on message "Ref" changed name from "typeParameters" to "type_parameters".
backend/protos/xyz/block/ftl/v1/schema/schema.proto:350:3:Field "3" with name "status" on message "VerbRuntime" changed type from "xyz.block.ftl.v1.schema.Status" to "xyz.block.ftl.v1.schema.VerbStatus".

None of these changes should effect wire compatibility.

Fixes #2935