Fix incorrect field types in several request structs

There are 3 fields in 2 structs that are of the type `Option>`, which is incorrect. It should be possible for them to contain a native asset as well. The affected structs:

- `all_claimable_balances_request`: field `asset`;
- `all_offers_request`: fields `selling` and `buying`.

The above asset type can be confusing, because we already have an `AssetType` in our request models. This type, however, can be of 3 types: native, alphanum4, and alphanum12. In some cases, an asset can be of 2 types: native and issued, which is the case for this issue.

We should introduce this new asset type in our request models. We should also check the rest of the codebase if the asset type with 2 options occurs in more request structs, and have them implement the new type.