Add support for jackson field ids

[brenbar]

Background

Jackson core has interfaces for field ids. This is a great opportunity for msgpack, since the protocol allows for integer keys, enabling more advanced binary serialization strategies for further reduced message size.

Current implementation of msgpack-jackson only allows for coercing strings to integers. Implementing the formal interfaces will enable end-to-end map serialization with mixed string/integer keys. Other msgpack implementations already support this.

Summary of Changes

1. Implement the formal jackson core interface for writing integer keys as field ids instead of strings.
2. Add a helper function on the msgpack parser that can be used in a jackson KeyDeserializer to deserialize field ids.

[brenbar]

@komamitsu @xerial Can CI be run on this PR to show the failing test? Then I'll follow-up with the implementation changes.

[brenbar]

@komamitsu @xerial Another CI run please 🙏

[brenbar]

🔨 Build failed with test showcasing new edge case.

[info] Test org.msgpack.jackson.dataformat.MessagePackDataformatForFieldIdTest.testMixedKeys started
[error] Test org.msgpack.jackson.dataformat.MessagePackDataformatForFieldIdTest.testMixedKeys failed: java.lang.AssertionError: expected: java.util.HashMap<{1=one, 2=two}> but was: java.util.HashMap<{1=one, 2=two}>, took 0.258 sec
[error]     at org.msgpack.jackson.dataformat.MessagePackDataformatForFieldIdTest.testMixedKeys(MessagePackDataformatForFieldIdTest.java:105)
[error]     ...
[info] Test run org.msgpack.jackson.dataformat.MessagePackDataformatForFieldIdTest finished: 1 failed, 0 ignored, 1 total, 0.264s

Will now demonstrate passing test with implementation changes.

[brenbar]

@komamitsu @xerial This is ready for another CI run to showcase the new test passing.

[brenbar]

@komamitsu @xerial For backwards compatibility, I imagine we need some sort of feature flag that defaults to the prior implementation. Do you guys have any suggestions for how that flag should be defined? Should I just use the MessagePack.PackerConfig, or should I pass it directly from the MessagePackFactory to the MessagePackGenerator?

Suggested change

	addKeyNode(id);
	if (writeIntegerMapKeysAsStringKeys) {
	super.writeFieldId(id);
	} else {
	addKeyNode(id);
	}