Merge bitcoindevkit/bdk#2099: Fix no-std compilation

Merge bitcoindevkit/bdk#2099: Fix no-std compilation

546b07e23b6219693976d7a4eeaacdc7c304f04a chore(example_cli,esplora): Fix no-std compilation (志宇)

Pull request description:

  ### Description

  `serde_json` requires either `std` or `alloc` feature enabled. We just enable `alloc` as we don't need the features that `std` provides.

  ### Notes to the reviewers

  `alloc` is just a subset of `std`. `json_serde` has `from_reader()` & `to_writer()` methods if `std` is enabled. We aren't using those methods so just having `alloc` is fine.

  I'm also not sure why an explicit `alloc` cargo feature flag is needed in `serde_json`. They can just have an internal prelude based on whether the `std` feature is enabled.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  luisschwab:
    ACK 546b07e23b6219693976d7a4eeaacdc7c304f04a
  oleonardolima:
    ACK 546b07e23b6219693976d7a4eeaacdc7c304f04a

Tree-SHA512: 05afc4d7ee3c1c7de8f2cd497a6a02f7bc4f1410e83604627c8af2b1a36fb92cd2fbe774a8d63bf2c4cffc8d4ddcc009adfdaeb59cdca3201f128f34639f39c2