for int64 type is string but format field is not generated

"int64" is intentionally converted as string in spec generation but format field is not generated at all. For other types like enum, when it is converted as string, format field is generated to indicate its true type. Same should be done for int64.

Example:
cmd/protoc-gen-openapi/examples/tests/protobuftypes/openapi.yaml:446
int64_value_type:
type: string

It should be:
int64_value_type:
type: string
format: int64

Start with cmd/protoc-gen-openapi/examples/tests/protobuftypes/openapi.yaml around line 446 and inspect the generated schema for int64_value_type. Trace the int64 handling in the protoc-gen-openapi generation path, then regenerate or run the relevant example tests; done means the schema includes type: string and format: int64 for int64 values.

Written by the indexing model from the issue text.