Support asset field in BaseTransaction

[MaciejBaj]

Expected behavior

BasicTransaction should store the asset property and implement the default behaviour for assetToJSON function, so the transactions don't necessarily need to implement the following constructor:

public constructor(rawTransaction: unknown) {
		super(rawTransaction);
		const tx = (typeof rawTransaction === 'object' && rawTransaction !== null
			? rawTransaction
			: {}) as Partial<TransactionJSON>;
		// Initializes to empty object if it doesn't exist
		this.asset = (tx.asset || {}) as TransferAsset;
}

All of the transaction types implement the assetToJSON function in the same way, which also can be moved to the BasicTransaction:

	public assetToJSON(): TransferAsset {
		return this.asset;
	}

Which version(s) does this affect? (Environment, OS, etc...)

2.0