Update JSON.NET code to work with Unity.

[pmaytak]

One of the possible solutions to #2343.

Adds a Unity preprocessor flag. Updates the JSON.NET code to use late bound serializers for Unity.

• Make code changes
• Run perf tests before & after change.

Perf test results
NetCore3.1 before changes:

Method	Mean	Error	StdDev
Serialize_MsalTokenResponse_Test	7.819 μs	0.0984 μs	0.0920 μs
Deserialize_MsalTokenResponse_Test	12.899 μs	0.1522 μs	0.1423 μs
Serialize_InstanceDiscoveryResponse_Test	41.662 μs	0.0342 μs	0.0267 μs
Deserialize_InstanceDiscoveryResponse_Test	72.998 μs	0.8727 μs	0.7288 μs
Serialize_OAuth2ResponseBase_Test	2.213 μs	0.0152 μs	0.0134 μs
Deserialize_OAuth2ResponseBase_Test	3.852 μs	0.0332 μs	0.0295 μs

NetCore3.1 after changes:

Method	Mean	Error	StdDev
Serialize_MsalTokenResponse_Test	8.112 μs	0.1621 μs	0.1991 μs
Deserialize_MsalTokenResponse_Test	13.649 μs	0.2716 μs	0.2789 μs
Serialize_InstanceDiscoveryResponse_Test	42.060 μs	0.5941 μs	0.5267 μs
Deserialize_InstanceDiscoveryResponse_Test	74.561 μs	1.1682 μs	0.9755 μs
Serialize_OAuth2ResponseBase_Test	2.240 μs	0.0425 μs	0.0398 μs
Deserialize_OAuth2ResponseBase_Test	4.008 μs	0.0425 μs	0.0398 μs

Net472 before changes:

Method	Mean	Error	StdDev
Serialize_MsalTokenResponse_Test	4.139 μs	0.0195 μs	0.0163 μs
Deserialize_MsalTokenResponse_Test	8.706 μs	0.0936 μs	0.0830 μs
Serialize_InstanceDiscoveryResponse_Test	28.913 μs	0.3980 μs	0.3528 μs
Deserialize_InstanceDiscoveryResponse_Test	46.775 μs	0.6654 μs	0.6224 μs
Serialize_OAuth2ResponseBase_Test	1.566 μs	0.0201 μs	0.0167 μs
Deserialize_OAuth2ResponseBase_Test	2.635 μs	0.0167 μs	0.0148 μs

Net472 after changes:

Method	Mean	Error	StdDev
Serialize_MsalTokenResponse_Test	7.405 μs	0.0803 μs	0.0712 μs
Deserialize_MsalTokenResponse_Test	13.201 μs	0.0551 μs	0.0488 μs
Serialize_InstanceDiscoveryResponse_Test	41.115 μs	0.5279 μs	0.4408 μs
Deserialize_InstanceDiscoveryResponse_Test	78.641 μs	0.4463 μs	0.4175 μs
Serialize_OAuth2ResponseBase_Test	2.341 μs	0.0135 μs	0.0120 μs
Deserialize_OAuth2ResponseBase_Test	4.302 μs	0.0077 μs	0.0068 μs