Merge bitcoindevkit/bdk#526: Add code example for each supported backend

f99a6b9f434e86a78695da72e9d28e6937c7e5db add `esplora_backend` example. (w0xlt)
aedbc8c97d6bf87347c3fd3796d1f0dfe6a8701c add `electrum_backend` example. (w0xlt)

Pull request description:

  This PR adds code example for connecting to Esplora, Electrum Server, Neutrino and Bitcoin Core.
  Also shows how to retrieve balance, sign and broadcast transactions.

  To test:
  ```
  cd examples/backend/
  cargo run electrum
  cargo run esplora
  cargo run neutrino
  cargo run rpc_core
  ```

ACKs for top commit:
  rajarshimaitra:
    tACK f99a6b9f434e86a78695da72e9d28e6937c7e5db

Tree-SHA512: 1d99129f14d83d9a833cee1587fe0eb3e5da4c83ae9008fb3e510be96a874dc86f800f203f68f5da70648a911709107cf0f286c2623808dc97dd63b7addef16b