Replace JsonWebToken ReadPayloadValue with a delegate

[pmaytak]

#2982

[pmaytak]

Looks like when using delegates there're extra allocations because of:

string claimName = reader.GetString();
claims[claimName] = ReadTokenPayloadValueDelegate(ref reader, claimName);

Delegates	Method	Mean	Ratio	Error	StdDev	P90	P95	P100	Gen0	Allocated	Alloc Ratio
Without	JsonWebTokenHandler_ValidateTokenAsyncWithTVP	30.72 μs	1.000	0.026 μs	0.055 μs	30.79 μs	30.81 μs	30.89 μs	0.2441	7.23 KB	1.00
With	JsonWebTokenHandler_ValidateTokenAsyncWithTVP	30.66 μs	0.998	0.124 μs	0.273 μs	30.96 μs	31.02 μs	31.10 μs	0.3052	7.55 KB	1.04

Delegates	Method	Mean	Ratio	Error	StdDev	P90	P95	P100	Gen0	Allocated	Alloc Ratio
Without	ReadJWS_FromMemory	8.373 μs	1.00	0.0331 μs	0.0727 μs	8.465 μs	8.499 μs	8.530 μs	0.2289	5.84 KB	1.00
With	ReadJWS_FromMemory	8.201 μs	0.98	0.0287 μs	0.0636 μs	8.295 μs	8.313 μs	8.357 μs	0.2441	6.07 KB	1.04