Implement and use batch version of `transaction.get_merkle` in `electrum-client`.

[oleonardolima]

I guess the batch version of transaction.get_merkle could be implemented as an API method in electrum-client.

Originally posted by @ValuedMammal in #1957 (comment)

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As mentioned in the comment above, the bdk_electrum could use a batch version of transaction.get_merkle method, once it's added there.

• add batch_transaction_get_merkle to rust-electrum-client. feat: add batch_transaction_get_merkle rust-electrum-client#170
• bump electrum-client and update bdk_electrum to use the new batch version. chore(bdk-electrum): use new batch_transaction_get_merkle API #1989