feat(iroh-sync): read only replicas

[divagant-martian]

Description

Closes #1767
Updates sync, the sync db, commands and tickets to enable read only replicas

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.

[Frando]

Looks good! Couple of minor nits in the review comments.

One question though:
If reading the current code correctly, importing a read capability (e.g. from a ticket) for an existing, writable namespace would delete a namespace's secret key. This is not the desired outcome, I'd say. What should happen IMO:

• Import write capability for existing namespace with read capability: Upgrade to write capability
• Import read capability for existing namespace with write capability: No change (but also no error)

Would be great to change this and add a test.

[Frando]

I added code to merge capabilities. When importing a write capability for a namespace for which you have a read-only capability currently, it will now merge the capabilities in the store. however, if an actual Replica instance is opened in the sync actor, it will not yet update the capability to write, which is a problem. How should we solve this? Currently, the store does not keep a copy of the replica (it is non-cloneable, this was changed in the actor pr).

What we could do is that the actor would both do store.import_namespace and replica.update_capability or so. wdyt?

[Frando]

I pushed another commit that takes care of upgrading capabilities of open replicas.