Extend RSAlgorithmFactory to support all ctors of RS256Algorithm

[flerka]

This is my draft implantation for #220

[abatishchev]

Hi, thanks for the contribution!
Can you explain the idea behind heaving Func over instantiating an algorithm directly?

[flerka]

This was done to avoid confusing if inside switch statement. All Func can be rewritten with:

            switch (algorithm)
            {
                case JwtHashAlgorithm.RS256:
                    {
                        if (_publicKey != null)
                        {
                            return new RS256Algorithm(publicKey);
                        }

                        var certificate = _certFactory();
#if NETSTANDARD1_3
            return new RS256Algorithm((RSACryptoServiceProvider)certificate.GetRSAPublicKey(), certificate.GetRSAPrivateKey());
#else
                        return new RS256Algorithm((RSACryptoServiceProvider)certificate.PublicKey.Key, (RSA)certificate.PrivateKey);
#endif

                    }
                default:
                    throw new NotSupportedException($"For algorithm {Enum.GetName(typeof(JwtHashAlgorithm), algorithm)} please use the appropriate factory by implementing {nameof(IAlgorithmFactory)}");
            }

If this is better from your point of view, I can rollback previous changes and commit this. What should I do @abatishchev ?

[abatishchev]

I was thinking more about this factory and realized we might not need it at all. Since it supports a single algorithm anyway. So it just mimics the algorithm's own constrictors. Does it add any value in top of it?
The initial ideas was to support multiple algorithms, help to instantiate one or another. But that has never happened.
So I'd remove it together. What do you think?

[abatishchev]

I'd remove JwtHashAlgorithm as well. Wondering what's the percentage of the users of this library uses symmetric algorithms still. Hope that not much.

[abatishchev]

Actually I got your idea and I like it. It's better than mine in #224. Can you please rebase on the latest master?

[abatishchev]

@flerka thanks for your contribution, much appreciated.