This PR introduces copying agents feature in the Library. Users can copy
and download their library agents but they can edit only the ones they
own (included copied ones).
### Changes 🏗️
- DB migration: add relation in `AgentGraph`: `forked_from_id` and
`forked_from_version`
- Add `fork_graph` function that makes a hardcopy of agent graph and its
nodes (all with new ids)
- Add `fork_library_agent` that copies library agent and its graph for a
user
- Add endpoint `/library/agents/{libraryAgentId}/fork`
- Add UI to `library/agents/[id]/page.tsx`: `Edit a copy` button with
dialog confirmation
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Agent can be copied, edited and runs
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