Add C# signing example using Bouncy Castle library

[psqli]

No description provided.

[tresf]

@pasqualirb thanks kindly for the code snippet. There's a few areas I'd like changed to closer match our examples. I've made some comments. If not addressed, that's OK, we would supercede them in a separate PR.

[psqli]

@pasqualirb thanks kindly for the code snippet. There's a few areas I'd like changed to closer match our examples. I've made some comments. If not addressed, that's OK, we would supercede them in a separate PR.

I'll make the changes and as soon as possible I rebase this PR.

[psqli]

I rebased the PR with the requested changes. Please tell me if you find a problem or if you would like any other change.

[tresf]

This looks great, thank you! I think we can eventually fix our examples to cache the private key on each sign request, but that problem already exists in our other .NET examples so that can be tackled at another time. Thanks!!