Unity Scripting Define Symbol

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# Feature Request
Add a versioned scripting define symbol such as `METAMASK_{Version Number}` to the Unity project settings when Metamask Unity is installed.

## Why it is needed
1. This allows developers to conditionally compile code for tools that are designed with an optional feature to use Metamask Unity.
2. Scripts made using these scripting defines can be more easily upgraded to support the latest Metamask Unity version.

## Possible implementation
Add a csc.rsp file in Assets/ with contents
```
-define:METAMASK_V1
```

### Code sample
Code can be conditionally compiled when Metamask is included in the Unity project. The version should be increased any time there is major changes.
```
#if METAMASK_V1
//Metamask version 1 specific code
#elif METAMASK_V2
//Metamask version 2 specific code
#endif
```
### Example
This example script shows usage of a manually added METAMASK scripting define symbol. The script can exist in the project, without compilation errors, when Metamask Unity is not installed and the METAMASK scripting define has not been added.
https://github.com/WoodsFiend/Moralis-Unity-SDK/blob/master/Scripts/MetaMask/MetaMaskProxyServerLogin.cs

Start with the proposed Assets/csc.rsp file and review the Unity project settings behavior described in the issue. Done means installing Metamask Unity adds a versioned symbol such as METAMASK_V1, allowing the shown conditional compilation pattern to work without errors when the SDK is absent.

Written by the indexing model from the issue text.