Make LLMCharacter extendable

[3inary]

Describe the feature

Hi, i'd like to be able to define the json save (for ex. add a title) and to do so override the Load/Save functions.
To make this possible some signatures need to be modified in LLMCharacter:

protected string GetSavePath(string filename)
protected string GetJsonSavePath(string filename)
protected string GetCacheSavePath(string filename)
public/protected virtual async Task Save(string filename)
public/protected virtual async Task Load(string filename)
// protected async Task Slot(string filepath, string action)

And in LLMCharacterEditor:
[CustomEditor(typeof(LLMCharacter), true)] // set to true to accept child classes as well

[amakropoulos]

Sure, will do that for the next release 👍