BEEFY gadget should recognize BeefyApi updates

[adoerr]

Currently, the BEEFY gadget is using a somewhat crude approach in order to detect the availability of an on-chain BEEFY pallet. One situation which would cause some mayhem, for example, is the removal of the BEEFY pallet as part of a runtime update.

The canonical approach followed by the BEEFY gadget should actually be to follow runtime updates and explicitly check for the availability of the BeefyApi. This would require basically two things:

• catch and process frame_system::pallet::Event::CodeUpdated events
• calling either sp_api::ProvideRuntimeApi::runtime_api()::has_api() or has_api_with() and verify that the new runtime provides the BeefyApi.

[tomusdrw]

catch and process frame_system::pallet::Event::CodeUpdated events

Events are not really reliable - we would need to be sure that we always receive all of them, and it's not going to be a case unless we plug ourselves into block import pipeline.

I think using has_api is the right way to to do it - we can simply do it before EVERY call to the BeefyApi - it's cheap enough.

[adoerr]

Changed title of the issue, since we are actually interested in a version change of the BeefyApi.

Recently a function api_version() has been added to RuntimeVersion. This allows for the following approach:

• Use the #[api_version(x)] attribute supported by the decl_runtime_apis! macro in order to version BeefyApi. Note, currently we are at version 1, since this is the default, if no explicit version is provided.
• Use Client::runttime_version_at() to get the RuntimeVersion at a given block number and call api_version() in order to check the BeefyApi version.