Add "Kind" message enums

[daved]

AC

• [ ] Using PB enums (https://developers.google.com/protocol-buffers/docs/proto3#enum), add values for Ask/Offer as Kind (or better naming, please consult with the team)
• [ ] Drop: AddTypeReq
• [ ] Rename: TypeResp->KindResp, TypesResp->KindsResp, FndTypesReq->FndKindsReq
• [ ] Ensure that the database migrations include "Kind" column (investigate what this looks like, probably id/int stored in field)
• [ ] Add "Kind" enum: PostResp, AddPostReq, OvrPostReq, PostsResp, FndPostsReq,
• [ ] *Expand on these AC as needed (please reach out and update the team)