feat: Support format for 64-bit integer field types

[esraaghanemali]

In this PR I update generator to add a format property to the schema for the proto definitions: int64, sint64, uint64, sfixed64, and fixed64
As this was the case in OpenApi v2 specifications and we are migrating to openApi v3, we still need this info as we handle each one differently.

previous behavior
message MyMessage {
int64 my_field = 1;
}
the generated openApi V3 specifications are

components:
  schemas:
    MyMessage:
      type: object
      properties:
        my_field:
          type: string

but openApi v2 https://github.com/grpc-ecosystem/grpc-gateway/tree/main/protoc-gen-openapiv2 we got this

components:
  schemas:
    MyMessage:
      type: object
      properties:
        my_field:
          type: string
          format: int64

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[arthaali]

@timburks Hello, can you please take a look at this addition to the type int64