feat(multi-tenant): Add option to pass a filter function for tenants array

[simovicaleksa]

Add option to pass a filter function to the tenants array/select. My use case for this would be so superadmin users can only edit specific tenants on their respective domains.

So for example in application with 3 tenants (e.g. A, B and C):

Domain	Has Access To
a.example.com	A
b.example.com	B
admin.example.com	A, B, C

Of course this is just one use case that I've came up with, but theoretically you could come up with as many, or as few custom rules as you'd like.

Example of such configuration in payload.config.ts would look something like this:

// payload.config.ts
plugins: [
    multiTenantPlugin({
      collections: {
        // ...
      },
      userHasAccessToAllTenants: (user) => isSuperAdmin(user),
      filterTenants: async ({req, tenants}) => {
        const hostDomain = await getHostDomain(req);
        if (hostDomain === "admin.example.com") return tenants;

        const filteredTenants = [tenants.find((tenant) => tenant.domain === hostDomain)]

        return filteredTenants;
      }
    }),
  // ...
]

As far as I can see the only way to implement something like this would be to create a custom component for the tenant selection which is far from ideal.