`in-person` signer type is unusable through the dotnet interface

[Hasan-75]

I was working with the dotnet-anvil library recently.
I tried to use the in-person signer type using the type EtchSigner.cs in the dotnet interface.
But I failed due to the restrictive implementation of the type.

if (value.ToLower() == "email" || value.ToLower() == "embedded")
{
    _signerType = value;
}
else
{
    throw new ArgumentException("SignerType must be either `email` or `embedded`");
}

I finally made my code work by switching to direct GraphQL call.
But it will be great if the in-person signer type could be made usable via the interface.

On a related note:

Do you accept open source contribution?
If so, I would like to make some contribution in the dotnet-anvil repo along with a fix for this issue.