Harmonise and explicit graphql query types

I disagree, we can probably derive a sensible name from the `graphql_type` + fields we are trying to fetch. Having `TxBlockCheckpoint` and `TransactionBlockCheckpoint` is not acceptable. But this probably means changing the whole file to harmonise so I'll open an issue for later.

_Originally posted by @thibault-martinez in https://github.com/iotaledger/iota-rust-sdk/pull/349#discussion_r2480700478_