[feature] Programmatic model builder

[nodece]

Background

Casbin models are currently defined as plain text (model.conf).
While flexible, this makes it inconvenient to analyze, refactor, or compose models programmatically, especially in larger or long-lived projects.

This proposal explores adding a programmatic way to construct models, without changing existing semantics or model behavior.

Proposal

Introduce an optional model builder that allows Casbin models to be constructed in code and then converted into a standard Casbin model.

Example (conceptual):

model := builder.New().
    Request("sub", "obj", "act").
    Policy("sub", "obj", "act").
    Effect(builder.AllowOverride).
    Matcher(
        builder.And(
            builder.Eq("sub"),
            builder.Eq("obj"),
            builder.Eq("act"),
        ),
    ).
    Build()

The resulting model is equivalent to one loaded from model.conf.

Design Notes

• The builder does not replace model.conf
• Core enforcement logic remains unchanged
• Effect and matcher expressions are generated using existing Casbin semantics
• The builder provides a structured way to construct commonly used model patterns

Effect and matcher definitions may be derived from previously defined sections, such as request and policy fields, but are ultimately emitted as standard Casbin expressions.

Scope

In scope

• Programmatic construction of model sections
• Generating standard Casbin models

Out of scope

• Introducing a new DSL or matcher language
• Enforce-time validation or semantic changes

[hsluoyz]

don't add so many test files. only builder_test.go is enough. builder_test.go only needs to contain two test cases, which is the builder version of below RBAC and ABAC tests. (reuse testEnforce)

func TestRBACModel(t *testing.T) {
	e, _ := NewEnforcer("examples/rbac_model.conf", "examples/rbac_policy.csv")

	testEnforce(t, e, "alice", "data1", "read", true)
	testEnforce(t, e, "alice", "data1", "write", false)
	testEnforce(t, e, "alice", "data2", "read", true)
	testEnforce(t, e, "alice", "data2", "write", true)
	testEnforce(t, e, "bob", "data1", "read", false)
	testEnforce(t, e, "bob", "data1", "write", false)
	testEnforce(t, e, "bob", "data2", "read", false)
	testEnforce(t, e, "bob", "data2", "write", true)
}

func TestABACModel(t *testing.T) {
	e, _ := NewEnforcer("examples/abac_model.conf")

	data1 := newTestResource("data1", "alice")
	data2 := newTestResource("data2", "bob")

	testEnforce(t, e, "alice", data1, "read", true)
	testEnforce(t, e, "alice", data1, "write", true)
	testEnforce(t, e, "alice", data2, "read", false)
	testEnforce(t, e, "alice", data2, "write", false)
	testEnforce(t, e, "bob", data1, "read", false)
	testEnforce(t, e, "bob", data1, "write", false)
	testEnforce(t, e, "bob", data2, "read", true)
	testEnforce(t, e, "bob", data2, "write", true)
}

the new Go files' Apache header should be year 2026

the builder should have a ToString method to print the model text out