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

signer/wasm-tests/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "wasm-tests"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dev-dependencies]
+wasm-bindgen-test = "0.3.24"
+tracing-wasm = "0.2.1"
+console_error_panic_hook = "0.1.7"
+serde_json = "1"
+
+# This crate is not a part of the workspace, because we want to
+# enable the "web" feature here but don't want it enabled as part
+# of workspace builds. Also disable the "subxt" feature here because
+# we want to ensure it works in isolation of that.
+subxt-signer = { path = "..", default-features = false, features = ["web"]  }
+
+# this shouldn't be needed, it's in workspace.exclude, but still
+# I get the complaint unless I add it...
+[workspace]