Add XmlDSigVerifier wrapper for SignedXml #638

Job	Run time
Test Code (ubuntu-latest, latest, true)	1m 6s
Test Code (ubuntu-latest, 16, false)	1m 0s
Test Code (ubuntu-latest, 20, false)	1m 1s
Lint Code	20s
Test Code (ubuntu-latest, 18, false)	1m 2s
	4m 29s

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