feat: Add custom subField

[WilsonLe]

No description provided.

[marcusforsberg]

Hi @WilsonLe, thanks for this excellent plugin, it was a breeze to setup!

I just tried the subField option but from what I can see that config option isn't actually being applied anywhere. The below has no effect, except for it using googleId as the name instead of sub:

subField: {
    name: 'googleId',
    label: 'Google ID',
    type: 'text',
    index: true,
    admin: {
      hidden: true,
      access: {
        read: () => true,
        create: () => true,
        update: () => false,
      }
    }
},

I guess it's this part of modify-auth-collection.ts that should take subField into account:

 if (!existingSubField) {
    fields.push({
      name: subFieldName,
      type: "text",
      index: true,
      access: {
        read: () => true,
        create: () => true,
        update: () => false,
      },
});

Would you like me to open an issue and/or PR for this?

[WilsonLe]

@marcusforsberg Good catch, would be great to have an issue for this, thanks!