Add subxt_signer crate for native & WASM compatible signing (#1016)

* Add and use subxt-signer crate for WASM compatible signing

* cargo fmt

* dev keypairs already references

* WIP fix various breakages

* re-jig features to be simpler and various test fixes etc

* doc and web fix

* fix various bits and pieces

* fix a test I broke

* dev-deps can't be linked to in docs, hrmph

* cargo fmt

* another doc link

* document the subxt_signer crate more thoroughly

* move feature flag for consistency

* more docs, no default subxt feature flag on signer, update release instrs

* Add missing license header

* unwrap_inner => into_inner

* extend a test a little to better check derive junctions

* note more clearly that the crypto bits come from sp_core::crypto

testing/test-runtime/Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 publish = false
 
 [dependencies]
-subxt = { workspace = true }
+subxt = { workspace = true, features = ["native"] }
 
 [build-dependencies]
 substrate-runner = { workspace = true }