Implement RSASSA-PSS support

[MatthiasValvekens]

As discussed on Slack:

Regarding RSASSA-PSS: I managed to put together a working & reasonably user-friendly implementation that does both signing and verifying yesterday evening. Since I have no way of knowing which classes require manual porting, I can’t say for sure how autoportable my code is, but I think the only manual porting required is in a 3-line method in SignUtils since that one relies on a piece of JCA API.
The breaking changes are all quite minor and in parts of the code base that already have breaking changes anyway:

• An extra method in IExternalSignature to retrieve the parameters. I’d make it a default method that returns null (which would make the change non-breaking) but IIRC the autoporter can’t deal with that, right?

• A couple extra factory methods in the IBouncyCastleFactory and its implementations + some additional exposed methods in some of the wrapper interfaces.

The total size of the change is also relatively small (~500 LoC, a large portion of which is there because I needed a table of digest output sizes to generate reasonable default parameters, and of course there’s the wrapper overhead).

[glenner003]

This PR has been approved and merged:
8b900230f
67a6730b

9bd3822c
bda972d1

[MatthiasValvekens]

Closing this as it's been merged :)