Feature: allow for custom IOU (TrustLine) xApp binding (button)

[WietseWind]

Right now, if the curated assets are like this:

xapp_identifier = null

If the xApp identifier is not null:

... We render Buy & Sell buttons:

Addition

If xapp_custom is given, with:

• xapp_identifier
• title

We will render one (one) extra button. Either it'll be the only one, or combined with the regular xApp identifier, there will be three buttons.

• The xapp_custom button comes first.
• The icon will be the xApp icon
• The title will be as given
• It opens the given xApp with the same entry params as the existing buy/sell buttons

[WietseWind]

Goal: Teleporter/cross chain IOU swaps xApp