Add support for OneTimeUse assertions

[madaster97]

Describe the problem you'd like to have solved

Add support for the OneTimeUse SAML condition, as an option to signal to service-providers to only accept a specific assertion once (keyed by the ID in the assertion).

Describe the ideal solution

Add a true/false flag in config for onlyOneTimeUse, to selectively add the condition to the assertion

Alternatives and current work-arounds

Setting a small lifetimeInSeconds helps mitigate the same sort of issues that OneTimeUse does. See some examples in the OWASP SAML Security cheatsheet

PS - Should consider the importance of fixing #73 , since we use the crypto functions referenced there to create the ID value that is used to enforce the one-time-use condition