Detect binary extensions in types, and mark those fields as optional

[aaroncox]

If we detect a binary extension in the ABI, we need to mark those fields as optional.

{
    "name": "unlinkauth",
    "base": "",
    "fields": [
        {
            "name": "account",
            "type": "name"
        },
        {
            "name": "code",
            "type": "name"
        },
        {
            "name": "type",
            "type": "name"
        },
        {
            "name": "authorized_by",
            "type": "name$"
        }
    ]
}

The authorized_by field at the bottom has a $ at the end of its type, indicating its a binary extension, and these fields from my understanding are always optional. Currently our code generation isn't accounting for this, and is outputting:

@Struct.type('unlinkauth')
export class Unlinkauth extends Struct {
    @Struct.field(Name)
    account!: Name
    @Struct.field(Name)
    code!: Name
    @Struct.field(Name)
    type!: Name
    @Struct.field(Name)
    authorized_by!: Name
}

and

export interface Unlinkauth {
    account: NameType
    code: NameType
    type: NameType
    authorized_by: NameType
}

The authorized_by fields in both of these should be marked as optional.