Add support for extensible enum decoding

[vp-elitnet]

Trying to decode an enum with a @HasExtensionMarker annotation fails with the following error.

java.lang.UnsupportedOperationException: choice extension is not implemented yet
	at net.gcdc.asn1.uper.EnumCoder.decode(EnumCoder.java:71)
	at net.gcdc.asn1.uper.UperEncoder.decode2(UperEncoder.java:86)
	at net.gcdc.asn1.uper.UperEncoder.decode(UperEncoder.java:61)
	at net.gcdc.asn1.uper.EnumCoderTest.extensibleExtSmall(EnumCoderTest.java:30)

A couple caveats apply to the proposed change:

• only decoding is supported, but not encoding;
• an extended enum is returned as null.

Due to the second point above, if a protocol used an extensible optional enum, it would be impossible to tell whether an extended value was encoded or the optional enum was not encoded at all. Anyway, that is in my opinion better than not being able to decode such messages at all. And for protocols that do not use extensible enums there is no impact.

Another point to take care of is version change. If the project followed semver and used Maven Release Plugin (snapshot versions), I would change the version to 1.2.0-SNAPSHOT, but I don't know what the versioning policy is.

[alexvoronov]

At least the error message was correct: "not implemented yet" :-) Thanks for adding the implementation!

We can simply bump version to 1.2.0, as a release of this new feature.