Allow WSSecurity and WSSecurityCert to be used together

[flowpl]

This PR changes/adds the following funtionality:

• a new ISecurity implementation (WSSecurityPlusCert) that takes WSSecurity and WSSecurityCert and combines their results into one SOAP request in case you need to use <UsernameToken> and <Signature> at the same time.
• changes to the WSSecurityCert implementation to enhance an existing <Security> header instead of adding another one.
• replace the static Id="_0" reference on the Body with reference Ids generated by xml-crypto.
• a new idMode option in WSSecurityCert to control the reference id generation of xml-crypto.

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I just realized that the functionality provided here is very close to PR #1187. Just FYI.