feat: configurable request timeout for carddav sync

[pabzm]

Big federated setups may need a longer timeout, which they now can configure.

This is motivated by a debugging session, in which we had to raise the timeout but could do so only via a patch.

The default timeout length doesn't change with this PR.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)

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