feat(core, runtime): Builtin actors framework

[ekovalev]

Following the discussion on reworking the original approach to builtin actors (#3504), a new extensible framework for Runtime builtin actors is proposed.

A set of BuiltinActor's is defined as the pallet_gear_builtin_actor::Config trait associated type - a tuple consisting of custom types each of which must implement the pallet_gear_builtin_actor::BuiltinActor trait (similar to how migrations or signed extras are defined in the Runtime). Those can be pallets (if access to on-chain storage is required) or any other custom types.
To enable messages routing to a specific builtin actor, the latter must be registered with the pallet_gear::Config::<T>::BuiltinRouter associated type that implements the pallet_gear::builtin::BuiltinRouter trait and provides the lookup() function to derive the respective BuiltinId based on the message destination address (ProgramId).
This functionality is provided by the gear_builtin_actor pallet.

The BuiltinId must be unique across all actors: it is claimed by a builtin actor itself as the actor's type's associated constant. The gear-builtin-actor pallet provides the register_actor() function that would check if there are conflicting BuiltinId's. This should be the preferred way to register an actor during Runtime upgrades.

This PR includes:

• builtin actor framework implementation: new assoc. type in the Gear pallet and the GearBuiltinActor pallet
• unit tests (more can be add in the future)
• migration template to register new actors
• RPC to query actors' ProgramId's
• docs
• adjustment of the calculate_gas_info() for the case of a builtin actor.

Examples of actual "production" builtin actor (like staking-proxy one) are beyond the scope of this PR and will be added subsequently.

[breathx]

pallets/gear-builtin is still under review and will be submitted asap

[breathx]

pallets/gear-builtin/src/tests/* to be reviewed after suggested changes applied/discussed

[breathx]

Awesome and huge work done!