feat: embed adapter to local ent package

[MuZhou233]

BREAKING
DRAFT: This provides a basic idea of fixing #20

What changed

• Update ent version (update go version as a side-effect)
• Refactor ent/schema/casbinrule.go to mixin.go
• Refactor adapter.go to casbin.tmpl
• Move ent to examples/ent
• Re-run generate

How does it work

This idea is based on two ent features: Schema Mixin and Template

Mixin allows us embed adapter schema definition to ent package

import "github.com/casbin/ent-adapter"

type CasbinRule struct {
	ent.Schema
}

func (CasbinRule) Mixin() []ent.Mixin {
	return []ent.Mixin{
		template.CasbinRuleMixin{},
	}
}

Template allows us embed adapter logic to ent package

import "github.com/casbin/ent-adapter"

entc.Generate("./schema",
	&gen.Config{},
	entc.Extensions(&template.CasbinExtension{}),
)

By doing this, the adapter is embedded/injected to local ent client. So that we can have fully control of it including implement custom logic in a single transaction.