fix: Fixed issue of RpcTarget not being properly disposed

[michalChrobot]

Purpose of this PR

This PR aims to resolve a memory leak issue reported in #3852

The bug was a memory leak in NetworkAnimator on clients due to RpcTarget not being properly disposed.
The cause of it was that in NetworkAnimator.OnNetworkSpawn(), the code cast the result of RpcTarget.Group() to RpcTargetGroup. On clients, RpcTarget.Group() returns a ProxyRpcTargetGroup (not RpcTargetGroup), so the cast returned null. The ProxyRpcTargetGroup with its NativeList<ulong> (allocated with Allocator.Persistent) was then leaked.
The fix was to change m_TargetGroup to use the IGroupRpcTarget interface (which both RpcTargetGroup and ProxyRpcTargetGroup implement)

By using IGroupRpcTarget (the interface both implement), m_TargetGroup can now hold:

• RpcTargetGroup when running on server
• ProxyRpcTargetGroup when running on client

The code only uses interface methods (Add(), Clear(), Target), so it works correctly regardless of which concrete class is stored.

Jira ticket

https://jira.unity3d.com/browse/UUM-133278

Changelog

• Fixed: memory leak in NetworkAnimator on clients where RpcTarget groups were not being properly disposed due to incorrect type casting of ProxyRpcTargetGroup to RpcTargetGroup.

Documentation

N/A

Testing & QA (How your changes can be verified during release Playtest)

I confirmed that when using latest develop-2.0.0 the issue was reproducible in the project but is not reproducible when using this branch

Backports

N/A

[EmandM]

This is brilliant! Thanks for finding the fix!

[NoelStephensUnity]

Nice fix!