Introduce capabilities filtering for off-chain runtime calls. (#3454)

* Introduce capabilities filtering for calls.

* Bump impl version.

* Allow RichOffchainCall to only read offchain db.

* Fix code.

* Panic on invalid calls.

* Merge execution contexts and expose capabilities.

* Fix repr

* Re-enable keystore for offchain calls.

substrate/node/runtime/src/lib.rs

@@ -80,7 +80,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 151,
-	impl_version: 152,
+	impl_version: 153,
 	apis: RUNTIME_API_VERSIONS,
 };