Offchain-worker APIs stubs (#2615)

* WiP: HTTP Apis.

* Working on the API.

* Add docs, clean up the API.

* Expose ext_ stuff as well.

* Implement HTTP helpers for offchain sr-io.

* Remove HTTP stuff.

* Revert "Remove HTTP stuff."

This reverts commit 7cca029d6ae93c5849b50edfcc6d2c313ba3e5bf.

* HTTP apis.

* Additional offchain methods.

* Make it compile.

* Implement wasm-ext boundary of offchain methods.

* Add stubs for offchain stuff to prevent panics.

* Fix tests.

* Addres some more issues.

* Introduce typedef, use unsafe from_utf8

* Bump runtime version.

* Introduce error to distinguish deadline and io errors.

* Add local_storage_cas

* Some tests for offchain stuff.

* Address more grumbles.

* Fix tests compilation.

* Fix borked merge.

* Improve docs for expected return values from ext functions.

* Adding new sign/enrypt/decrypt APIs.

substrate/Cargo.lock

@@ -3195,6 +3195,7 @@ dependencies = [
  "parity-codec 3.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_version 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "sr-std 2.0.0",
+ "substrate-offchain 2.0.0",
  "substrate-primitives 2.0.0",
  "substrate-state-machine 2.0.0",
  "substrate-trie 2.0.0",
@@ -4235,6 +4236,7 @@ dependencies = [
  "futures 0.1.27 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "parity-codec 3.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "parking_lot 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "sr-primitives 2.0.0",
  "substrate-client 2.0.0",
  "substrate-consensus-common 2.0.0",