Add SetField method for the message & composite to support nested path

[Stashchenko]

Feature for this #383

Feature: Introduce composite subfields handling with SubfieldAccessor interface

Problem

The current implementation of Unset/Set for subfields works only when the field is a *field.Composite.
This makes it hard to support more complex composite specs that wrap or embed a Composite.

For example, we faced this situation:

type CompositeDE48Field struct {
    TCC *field.String
    *field.Composite
}

var (
    _ field.Field      = (*CompositeDE48Field)(nil)
    _ json.Marshaler   = (*CompositeDE48Field)(nil)
    _ json.Unmarshaler = (*CompositeDE48Field)(nil)
)

In this case, Unset and Set won't work because of the strict type check against *Composite.

---

Solution

Introduce an interface SubfieldAccessor which abstracts composite-like fields.

type SubfieldAccessor interface {
    SetSubfields(idPath string, value string) error
    SetSubfield(id string, value string) error
    UnsetSubfields(idPaths ...string) error
    UnsetSubfield(id string)
}

Then, instead of requiring *Composite, check against this interface:

composite, ok := f.(SubfieldAccessor)
if !ok {
    return fmt.Errorf("field %s is not a composite field and its subfields %s cannot be unset", id, path)
}

---

Benefits

• Makes composite handling more universal.
• Supports both *field.Composite and custom composite wrappers like CompositeDE48Field.
• Keeps the API backward compatible.
• Provides flexibility for more complex composite field definitions in the future.