Add `@JsonKey` annotation (similar to `@JsonValue`) for customizable serialization of Map keys

Describe the bug When serializing a map key, if the key’s type uses @JsonValue on one of its attributes, and if that attribute’s type uses @JsonValue on one of its own attributes, the second @JsonValue is ignored, and toString() is used instead.

Version information 2.10.0

To Reproduce

    class Inner {

        @JsonValue
        String string;

        Inner(String string) {
            this.string = string;
        }

        public String toString() {
            return "Inner(String="+this.string+")";
        }

    }

    class Outer {

        @JsonValue
        Inner inner;

        Outer(Inner inner) {
            this.inner = inner;
        }

    }

    public void test() throws Exception {
        Outer outer = new Outer(new Inner("key"));
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(outer)); // outputs "key", as expected
        System.out.println(mapper.writeValueAsString(Collections.singletonMap(outer,"value"))); // outputs {"Inner(String=key)":"value"}, expected {"key":"value"}
    }

[Anusien]

I’ve started working on this.

[CidTori]

Yep, for example : while serializing a map key, if there is a @JsonKey, use it (with chaining), else if there is a @JsonValue, use it (with chaining too), else use toString()?