feat(electrs): add `mempool/electrs` support under a new feature

[oleonardolima]

The crate already has support for blockstream/electrs Esplora API under the feature esplora_a33e97e1, which it's of great use for testing Esplora API, and it's heavily used in rust-esplora-client.

Another implementation of the same API (I'd guess) is developed and maintained by the mempool team, under mempool/electrs.

I think it'd be interesting and a great addition to also have support for it under a new feature, it'd allow to cross-test both APIs and make sure they still align, at least in rust-esplora-client, as the users can choose different implementations to rely on in a non-test scenario, allowing to represent that on the tests.

Any other thoughts about this, any shortcomings I didn't see? Let me know if it'd be a good addition to the crate.

[RCasatta]

I guess most of the program arguments are compatible with mempool/electrs, I would accept the changes with a new feature to support it.

It doesn't seem they release official binaries, so a new job in the "build electrs" CI needs to be added