feat(backend): Class to manage btc pending transactions (#2598)

# Motivation

We want to store pending btc transactions to disable users from making
transactions while there is one that hasn't finished.

In this PR, I introduce the module to manage the pending transactions.
It adds functionality to add, read and prune the pending transaction.

This struc will then be added to the heap, (hence that I put it inside a
directory `heap_state`) and used in the backend endpoints.

# Changes

* New class `BtcUserPendingTransactions` in module
`heap_state/btc_user_pending_tx_state`.
* Add new module to lib.

# Tests

* Test class functionality.