generic compose_extrinsic() function

[brenzi]

The current version of this library can't compose extrinsics for custom runtimes because there's a dependency clash between custom node_runtime and substrate master node_runtime.

This should be solvable along these lines:

• not depending on substrate's node_runtime crate with api-client
• using type arguments, so a depending crate can supply the custom node_runtime dependencies

in addition to generic extrinsics, we should provide very common extrinsic types with easy function calls, probably wrapping the dispatch of the call as well:

• transfer() as in extrinsics.rs
• ink contract handling #5
• upgrade chain runtime as in Gav's Demo

write an example for how to compose an extrinsic for a custom runtime module using Gav's Demo: https://substrate.dev/docs/en/tutorials/creating-your-first-substrate-chain#step-6-interacting-with-our-new-module

[clangenb]

as well offer the option to use Sr25519 or Ed25519 keys

[clangenb]

Upgrade chain runtime has not been integrated. Rest has been implemented