cmd/sign: make it possible to specify additional extended key usage

[krisztian-kovacs]

Certain Microsoft services (like Remote Desktop Services) require certificates with special Extended Key Usage values. Microsoft has a registered OID (1.3.6.1.4.1.311.54.1.2) for "Remote Desktop Authentication":

http://oid-info.com/cgi-bin/display?oid=1.3.6.1.4.1.311.54.1.2&action=display

This change makes it possible to add an additional custom Extended Key Usage value to the signed certificate by allowing the user to specify an OID value explicitly which is then added to the list.

[CLAassistant]

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