srml-contract: update ext_random(_seed) (#2635)

* Initial implementation.

* Rename random_seed to random

* Update rustdocs

* Update COMPLEXITY.md

* Fix comment.

* Limit the size of subject.

* Bump the runtime version.

* Fix doc

* Update node/runtime/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>