Add Span overload(s) to TokenHandler

[BrennanConroy]

While benchmarking an ASP.NET Core app we found that ~5% of allocations come from a substring call to remove "Bearer " from the access token provided in the "Authorization" header. We can avoid this substring call if TokenHandler.ValidateTokenAsync had a ReadOnlySpan<char> overload.

azure-activedirectory-identitymodel-extensions-for-dotnet/src/Microsoft.IdentityModel.Tokens/TokenHandler.cs

Line 66 in 286d780

public virtual Task<TokenValidationResult> ValidateTokenAsync(string token, TokenValidationParameters validationParameters) => throw new NotImplementedException();

ASP.NET Core calls
https://github.com/dotnet/aspnetcore/blob/415cf1ec6d004648c27ab78359fd57d20c7c9797/src/Security/Authentication/JwtBearer/src/JwtBearerHandler.cs#L86
https://github.com/dotnet/aspnetcore/blob/415cf1ec6d004648c27ab78359fd57d20c7c9797/src/Security/Authentication/JwtBearer/src/JwtBearerHandler.cs#L107

Other methods like TokenHandler.ReadToken might also benefit, but I haven't analyzed them.

[brentschmaltz]

@BrennanConroy this PR is the first step, #2458

Once that is in, we plan on adding similar API's to JsonWebTokenHandler.ValidateToken(Span<>...)