Update JsonWebToken to enable extensibility

[pmaytak]

Fixes #2581.

Extract token reading code into a separate ReadPayloadValue so it can be overridden with custom logic in child classes.
Make ReadPayloadValue private protected (accessible by types derived from the containing class, but only within its containing assembly or friend assemblies).

Example derived class:

    public class CustomJsonWebToken : JsonWebToken
    {
        public CustomJsonWebToken (string jwtEncodedString) : base(jwtEncodedString) { }

        /// <summary>
        /// Tries to read custom claims first.
        /// If nothing matched, reads common claims and the rest of unnamed claims.
        /// </summary>
        private protected override void ReadPayloadValue(ref Utf8JsonReader reader, Dictionary<string, object> claims)
        {
            if (reader.ValueTextEquals("CustomClaim"))
            {
                _customClaim = JsonSerializerPrimitives.ReadString(ref reader, "CustomClaim", ClassName, true);
                claims["CustomClaim"] = _customClaim ;
            }
            else
            {
                base.ReadPropertyValue(ref reader, claims);
            }
        }

        private string _customClaim;

        public string CustomClaim
        {
            get
            {
                _customClaim ??= Payload.GetStringValue("CustomClaim");
                return _customClaim;
            }
        }
    }

There was no change in performance.