Reduce allocations and transformations when creating a token.

[brentschmaltz]

The purpose of this PR is to reduce the memory allocations when creating a JWS.
Strategy:

1. Use ArrayPools whenever possible.
2. Use a single MemoryStream at the highest level and make all writes into that stream.
3. For .net 6+, add new API's to SignatureProviders to accept memory buffer to put signatures.
4. Make use of Encoding.GetBytes that take a memory stream.

With these changes benchmark shows a slightly faster creation with less than half the memory.

For 1000 token creations using SecurityTokenDescriptor 2.48MB vs. 7.2MB.

Additional work for composing tokens where the memory buffer is passed to CreateToken.

[keegan-caruso]

Just a few small comments