Add "Sign" button for signing XML with contract key

[miang719]

This is needed before the XML can be uploaded/commited to the repo.

I imagine you need to use the xmlsectool, e.g. by including the jar or add it as dependency if it is in maven or gradle repo.

Be careful, in RFC 4050 you need to specify the right curve.

[lyhistory]

ok, will do it

[lyhistory]

just for my own reference, as discussed with Mr Weiwu over phone last Sunday, based on ABC principle, for now I should only include RFC4050 ECDSAKeyValue into xml (remove leading trailing extra whitespace) for next year, x509 sign using xmlsectool