feat: add managed reverse edge support

[matthewmcneely]

This PR allows defining reverse edges on parent structs using json:"~predicate" combined with dgraph:"reverse". On mutation, dgman automatically creates forward edges on children pointing to the parent.

Changes:

• schema.go: Add ManagedReverse/ForwardPredicate fields, detect ~predicate + reverse tag
• mutate.go: Create forward edge mutations on children for managed reverse edges
• query.go: Generate explicit reverse edge blocks in All() queries

Usage:

type Department struct { 
    Courses []*Course `json:"~in_department,omitempty" dgraph:"reverse"`
}
type Course struct { 
    InDepartment *Department `json:"in_department,omitempty" dgraph:"reverse"`
}

Mutating Department with embedded Courses now automatically sets Course.in_department edges, and queries expand reverse edges explicitly.

See the new reverse_test.go for all the capabilities