Merge bitcoindevkit#373: refactor: rename Wallet::indexed_graph field

4b77231 refactor: rename Wallet indexed_graph field to tx_graph (codingp110)

Pull request description:

  ### Description

  Renamed `Wallet::indexed_graph` to `Wallet::tx_graph` as the latter is a better name representing the field.

  ### Notes to the reviewers

  This PR helps with the multi-keychain work being done in bitcoindevkit#318 and bitcoindevkit#367 .

  ### Changelog notice

  ### Changed

  - Renamed `Wallet::indexed_graph` to `Wallet::tx_graph`.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `just p` before pushing

ACKs for top commit:
  ValuedMammal:
    ACK 4b77231

Tree-SHA512: bef7e576765efa965e952fe0335f4bd1cc31bfbc2ce9bd27ae1a4084a8515d2b6b5b00cdf439f766e2b932f9d0f9ffde8e148dfb960ae014c2d4b5d17658a55a

Author: ValuedMammal