Braintree 4.2.0

* Add support for VCR compelling evidence dispute representment * Remove non-CLS compliant `generate` and `generateAsync` methods from `ClientTokenGateway`. Use `Generate` or `GenerateAsync`. * Add `SubscriptionDetails` class and change `Transaction.Subscription` property to `Transaction.SubscriptionDetails`. * Add `CustomerDetails` class and change `Transaction.Customer` property to `Transaction.CustomerDetails`. * Remove `RefundId` from `Transaction`. Use `RefundIds` instead. * Remove `TransparentRedirectURLForCreate`, `TransparentRedirectURLForUpdate`, and `ConfirmTransparentRedirect` from `CreditCardGateway`, `CustomerGateway`, and `TransactionGateway`. Use `gateway.TransparentRedirect.Url` or `gateway.TransparentRedirect.Confirm()` instead. * Remove `CUSTOMER_ID_IS_INVAILD`, `TRANSACTION_LINE_ITEM_DISCOUNT_AMOUNT_MUST_BE_GREATER_THAN_ZERO`, and `TRANSACTION_LINE_ITEM_UNIT_TAX_AMOUNT_MUST_BE_GREATER_THAN_ZERO`, and `TRANSACTION_MERCHANT_ACCOUNT_NAME_IS_INVALID`. Use `CUSTOMER_ID_IS_INVALID`, `TRANSACTION_LINE_ITEM_DISCOUNT_AMOUNT_CANNOT_BE_NEGATIVE`, `TRANSACTION_LINE_ITEM_UNIT_TAX_AMOUNT_CANNOT_BE_NEGATIVE`, or `TRANSACTION_MERCHANT_ACCOUNT_ID_IS_INVALID` instead. * If an `AccessToken` and `Environment` are specified in `Configuration`, an exception will be thrown if the environment does not match the access token's environment. * For `RangeNode` decimal values passed to `SearchCriteria`, use `.ToString(InvariantCulture)` to prevent localization issues (https://github.com/braintree/braintree_dotnet/issues/68). * Change `OAuthGateway.ComputeSignature` to be an internal method. * Migrate the xproj build files to MSBuild csproj build files and update Dockerfile images for support.