feature: deploy custom tokens (if configured)

[prestwich]

Adds a deploy-time custom representation feature. This allows us to pre-define custom reprs for migration-less changeover later. To allow maximum flexibility, each representation is deployed with its own UBC and beacon. These reprs are set to delegate to the BridgeToken deploy initially.

To use:

• add an array of custom specifiers to the bridge config along weth in the chain configs
• the custom specifier is a domain + an address o 32-byte ID

e.g.

{ 
    customs: [
        {domain: 3000, id:"0xabcd.......1234", name: 'My Token', symbol: 'MYTOK', decimals: 8},
    ]
}