Improve feature combinations for ureq/reqwest

Our features are a bit convoluted, most annoyingly we cannot build with
`--all-features`. However we can make life for users a little easier.

Explicitly we want users to be able to:

- Use async-interface/WASM without using esplora (to implement their own blockchain)
- Use esplora in an ergonomic manner

Currently using esplora requires either reqwest or ureq. Instead of
making the user add all the features manually we can add features that
add the required feature sets, this makes it easier for users to
understand what is required and also makes usage easier.

With this patch applied we can do

- `cargo check --no-default-features --features=use-esplora-reqwest`
- `cargo check --no-default-features --features=use-esplora-ureq`
- `cargo check --features=use-esplora-ureq`
- `cargo check --no-default-features --features=async-trait`

src/lib.rs

@@ -223,9 +223,6 @@ compile_error!(
     "Features async-interface and compact_filters are mutually exclusive and cannot be enabled together"
 );
 
-#[cfg(all(feature = "esplora", not(feature = "ureq"), not(feature = "reqwest")))]
-compile_error!("Feature missing: esplora requires either ureq or reqwest to be enabled");
-
 #[cfg(feature = "keys-bip39")]
 extern crate bip39;