Add code example for each supported backend

[w0xlt]

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

[notmandatory]

Hi, please rebase to pickup changes in #596. Thanks!

[w0xlt]

@notmandatory sorry for the delay.

I force pushed a new version that adds examples just for Electrum and Esplora, as there are already examples for Compact Filter and RPC backends.

I also updated the code to 0.20.1 and addressed @rajarshimaitra's suggestion not to add a new cargo project.

[w0xlt]

The new push added the documentation suggested in https://github.com/bitcoindevkit/bdk/issues/685, explaining how to use use-esplora-reqwest.

[rajarshimaitra]

I don't know why CI / Rust fmt (pull_request) failed. I ran cargo fmt --all before committing.

It was some temporary problem with rust update server.. I restarted the tests and it passed..

[w0xlt]

Rebased. Is any additional action required ?