TokenPrice RuntimeApi and RPC Method

[mustermeiszer]

The api should provide the possibilities to call a node to retrieve the latest available token-price. This is a little tricky as calculating token-prices generally involves a "more" complex calculation.
It would be nice to have this anyways and this feature will most likely need us to provide a separate method for the pools pallet to achieve it.

• Should NOT fail when NAV is too old

Implementation Proposal

#[rpc]
pub trait TokenTranchePriceApi {
	#[rpc(name = "pools_tokenPrice")]
	fn tranche_token_price(&self, id: T::PoolId, tranche: TrancheLoc<T::TrancheId>) 
	    -> Result<BalanceRatio>;
}

decl_runtime_apis! {
    pub trait TokenTranchePriceApi {
	 fn tranche_token_price(id: T::PoolId, tranche: TrancheLoc<T::TrancheId>) -> Option<BalanceRatio>;
    }
}