feat: allow custom storage strategies via lifecycle hooks and custom readers

[EmaSchiavoni]

While Keystatic's internal persistence (local/GitHub) is excellent for most use cases, there are times when a project requires a specific collection to be managed externally due to custom business logic. Instead of forcing developers to leave the Keystatic ecosystem for these exceptions, this pull request introduces two complementary features that, together, allow for seamless integration into the same unified workflow:

1. Lifecycle Hooks (beforeSave, beforeDelete): New callbacks for collections and singletons. They provide full control over the persistence flow by receiving a save/delete function as an argument. This allows for side effects (logging, external sync) or complete overrides of the default behavior.
2. Custom Readers: A new reader property in the config that allows developers to define how data is fetched.

I know these could be two separate features, but I'm intentionally submitting them in a single PR because together they form a coherent extensibility tool: The hooks allow a "true" return value to indicate a successful operation even if Keystatic's internal save function wasn't called (for example, when saving to an external API). Without the custom reader, this would result in an inconsistent UI state where data is saved externally, but Keystatic can't retrieve it

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