Set `protocol` as string for `/evm/swaps`

[DenisCarriere]

Set protocol as string to allow other DEX protocols (bancor/curvefi, etc..) in /evm/swaps

[0237h]

Could we just expand the enum with the new protocols ?

[0237h]

One solution I can see also is similar to networks where we populate the enum field at API startup, that way we can fetch the list of protocols without having to manually update on new schema releases.