I'm currently migrating a database from an existing structure where we have used a variation of Snowflake to generate the IDs for our resources. The natural way to store such a value in the database would be an uint64/BIGINT UNSIGNED NOT NULL, thus a numeric type for ent.

In spite of there not being a straight DefaultFunc method on the uint64Builder, by getting the Descriptor I managed to set a custom function which is ran when the resource is created:

	fd := field.Uint64("id").Immutable().Positive()
	fd.Descriptor().Default = func() uint64 {
		// generate...
	}

However, once we try to run entc, the following error shows up:

2021/01/07 10:08:32 running ent codegen:unmarshal schema [... json data ...]: unexpected default value type for field: "id"

Tracing back the error, it is from this method:

https://github.com/facebook/ent/blob/d4e04cd08f76389f0db76adec1c62d3a48c36d7b/entc/load/schema.go#L354-L369

Being a numeric type and having a default, the type conversion is attempted, but f.DefaultValue will actually be of type nil because only DefaultFunc is set.

Proposals to solve this:

1. add a DefaultFunc check on the defaults() method, thus skipping it when we have such a function
2. or, the former + add full support by also adding DefaultFunc on all numeric type builders - although this entails expanding the API

I'd be happy to work on a PR for this - but wanted to check first what's the best path to follow here.