refactor(api)!: rename Tx to EsploraTx and clean up house

This commit:
  - Renames `Tx` to `EsploraTx`: more explicit. `Tx` is not precise about what it is.
  - Remove `EsploraTx::fee`, since the field is already public.
  - Implements `From<EsploraTx>`/`From<&EsploraTx>` to `Transaction` conversions.
  - Reorders structs into logical groups: Transaction, Block, Address, ScriptHash, UTXO, and Mempool.
  - Improves documentation.
  - Updates copyright notice to `2020-2026`.

Author: luisschwab