feat: Add Actor method use_state

[Mantisus]

Description

Add Actor method use_state. Allows the user to save data and states in kvs with periodic persistence.

Issues

Closes: #735

Testing

Added unit tests

[vdusek]

Let's please wait until the implementation is clarified in #735.

[vdusek]

LGTM

[Copilot]

Pull request overview

This PR adds a new use_state method to the Actor class, enabling users to easily create and manage state values that are automatically persisted to key-value stores. The state is persisted periodically (based on PERSIST_STATE_INTERVAL_MILLIS) and also when the Actor exits.

Changes:

• Added Actor.use_state() method for managing persistent state
• Implemented automatic state persistence on Actor exit via _save_actor_state()
• Added comprehensive unit tests covering default and custom configurations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/apify/_actor.py	Implements the use_state method, adds state store tracking, and integrates state persistence into Actor lifecycle
tests/unit/actor/test_actor_key_value_store.py	Adds unit tests for the new use_state functionality with various scenarios

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[vdusek]

quite good comments from copilot 🙂

[vdusek]

Ok, thanks for resolving those comments from Copilot, LGTM from my side, let's wait for @Pijukatel and merge it.