Add support for xmlsec package as backend

[joshuata]

Looking through the project history the decision to call out to the xmlsec1 binary made sense at the time due to the poor state of the library. However, the library looks to be in far better state now, with regular updates and a far more usable API, as well as pre-built wheels available for many architectures.

Using the library directly should offer both performance and safety benefits compared to the current setup, in addition to enabling new applications such as lambda/serverless deployments

[c00kiemon5ter]

Thank for filling in this issue. I agree that we should look into this.

[joshuata]

Is there anything I can do to help get the ball rolling on this?

[AndrewFoxUUID]

Is there anything I can do to help get the ball rolling on this?

A little late, but I made a fork and PR to hopefully integrate xmlsec better if you want to test it, https://github.com/IdentityPython/pysaml2/pull/961