Support network arg for adding signatures to transaction

[raymens]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[elucidsoft]

Looks like the test is failing:

Test method stellar_dotnet_sdk_test.TransactionTest.TestAddPreSignedSignature threw exception: 
stellar_dotnet_sdk.NoNetworkSelectedException: Exception of type 'stellar_dotnet_sdk.NoNetworkSelectedException' was thrown.

[raymens]

Thanks, I've updated it to default to Network.Current