Introduce separate type for pending blocks

[klkvr]

ref #1106 (comment)

Responses for pending blocks miss a bunch of fields, thus making it impossible to deserialize them without introducing many Options on RPC block types.

Possible solution could be to introduce a less restrictive type for pending blocks + separate methods for fetching those on Provider. After that we can disallow fetching pending blocks through provider.get_block_by_number, and use complete type for those responses.

[mattsse]

After that we can disallow fetching pending blocks through provider.get_block_by_number, and use complete type for those responses.

this could be pretty invasive because then we'd need a new BlockNumberOrTag call.

what we could also do is just deserialize the pending block response differently, I'm fine with using default for the missing fields, not super great but simpler than introducing a block type just for pending.

[klkvr]

what we could also do is just deserialize the pending block response differently

should we add a separate Network AT for pending block? like type PendingBlockResponse: RpcObject + Into<Self::BlockResponse>

and then in get_block_by_number deser as N::PendingBlockResponse if pending block is requested