Expand remote keystore interface to allow for hybrid mode (#7628)

* update to latest master

* updates on docs, license, meta

* hide ssrs behind feature flag

* implement remaining functions on the server

* sign server line length fix

* fix tests

* fixup in-memory-keystore

* adding failsafe

* skipping ecdsa test for now

* remote keystore param

* remote sign urls made available

* integrating keystore remotes features

* don't forget the dependency

* remove old cruft

* reset local keystore

* applying suggestions

* Switch to single remote, minor grumbles

* minor grumbles, docs

substrate/Cargo.lock

@@ -3967,6 +3967,7 @@ dependencies = [
  "sc-consensus-aura",
  "sc-executor",
  "sc-finality-grandpa",
+ "sc-keystore",
  "sc-rpc",
  "sc-rpc-api",
  "sc-service",
@@ -7484,6 +7485,7 @@ dependencies = [
  "merlin",
  "rand 0.7.3",
  "rand_core 0.5.1",
+ "serde",
  "sha2 0.8.2",
  "subtle 2.3.0",
  "zeroize",
@@ -8276,6 +8278,7 @@ dependencies = [
  "rand 0.7.3",
  "rand_chacha 0.2.2",
  "schnorrkel",
+ "serde",
  "sp-core",
  "sp-externalities",
 ]