Optional field builder should be covariant

[JacekLach]

Currently conjure generates builders for optionals like so:

        private Optional<Object> lt = Optional.empty();

        /** Optional exclusive upper bound for the value of the Field. */
        @JsonSetter("lt")
        public Builder lt(Optional<Object> lt) {
            this.lt = Objects.requireNonNull(lt, "lt cannot be null");
            return this;
        }

        /** Optional exclusive upper bound for the value of the Field. */
        public Builder lt(Object lt) {
            this.lt = Optional.of(Objects.requireNonNull(lt, "lt cannot be null"));
            return this;
        }

Since we know optionals are read-only, it's safe to be covariant here:

        private Optional<Object> lt = Optional.empty();

        /** Optional exclusive upper bound for the value of the Field. */
        @JsonSetter("lt")
        public Builder lt(Optional<? extends Object> lt) {
            this.lt = Objects.requireNonNull(lt, "lt cannot be null").flatMap(Optional::<Object>of);
            return this;
        }

        /** Optional exclusive upper bound for the value of the Field. */
        public Builder lt(Object lt) {
            this.lt = Optional.of(Objects.requireNonNull(lt, "lt cannot be null"));
            return this;
        }

which avoids a gotcha for Object specifically (passing Optional<String> into .lt and getting back Optional<Optional<String>>), and makes the interface nicer for all optionals.

[iamdanfox]

Closed by #9