frame: Expose on_genesis hook

[skunert]

This PR enables parachain onboarding directly in the initial session. Previously, we had to wait one full session before a parachain could produce blocks, which created unnecessary delays in zombienet tests during local debugging and CI runs.

Enhanced on_genesis Hook

A trait for executing code after genesis storage initialization already existed in frame:

polkadot-sdk/substrate/frame/support/src/traits/hooks.rs

Lines 162 to 165 in 6be4467

	pub trait OnGenesis {
	/// Something that should happen at genesis.
	fn on_genesis() {}
	}

However, this trait was not exposed to users through the Hooks trait and was only utilized internally by macros:

polkadot-sdk/substrate/frame/support/procedural/src/pallet/expand/pallet_struct.rs

Lines 220 to 229 in 6be4467

	// Implement `OnGenesis` for `Pallet`
	impl<#type_impl_gen> #frame_support::traits::OnGenesis
	for #pallet_ident<#type_use_gen>
	#config_where_clause
	{
	fn on_genesis() {
	let storage_version: #frame_support::traits::StorageVersion = #storage_version;
	storage_version.put::<Self>();
	}
	}

This PR adds an on_genesis method to the Hooks trait, allowing developers to define custom behavior that executes after each pallet has completed its genesis build process.

Session Handlers Now Called in on_genesis Hook

The previous inability to onboard parachains at genesis came from an initialization sequence issue. Parachain consensus-related pallets are initialized via the Initializer pallet here, which uses a SessionHandlers implementation. Previously, session handlers were called during the genesis build process, before parachain-related storage items were fully set up.

With this PR, session handlers are now called after the complete genesis state is established, enabling seamless parachain onboarding from the very first session.

[skunert]

/cmd prdoc --audience runtime_dev --bump major