generalize asset id in standard amm

[gregdhill]

Signed-off-by: Gregory Hill gregorydhill@outlook.com

Allows the consumer to use any AssetId when instantiating the "standard" AMM. In my case I would like to directly use the CurrencyId instead of converting to and from zenlink_protocol::AssetId since we anyway will have XCM disabled (see this comment). The important thing to note is that this PR does not introduce any functional changes since the consumer may still use the concrete AssetId type and PairLpGenerate.

[jerrybaoo]

1. It makes sense to keep the same format across all chains for cross-chain transactions. This is the reason why AssetId is not generic in the standard AMM.
2. Using trait to generate LPs is a meaningful change.
   We will make some improvements based on your PR

[jerrybaoo]

In fact, if the chain uses the substrate asset pallet(https://github.com/paritytech/substrate/tree/master/frame/assets), then the conversion is very smooth.

[gregdhill]

It makes sense to keep the same format across all chains for cross-chain transactions.

As I understand you anyway intend to separate the XCM functionality so I'm not convinced forcing this format across all chains is meaningful here - especially as the CurrencyId in the "stable" AMM is already generic.