feat: move primary object store configuration to a single place

[icewind1991]

Extracted from #52786 to get smaller, easier to review PRs

Extracts and unifies the handling of primary object store configuration.

As with #52786, this also allows configuring multibucket object storages by settings the multibucket arguments instead of having the configuration under the the objectstore_multibucket key. (it should have never be done like that tbh)

[icewind1991]

/backport to stable31

[icewind1991]

/backport to stable30

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31
# Create the new backport branch
git checkout -b backport/52816/stable31
# Cherry pick the change from the commit sha1 of the change against the default branch# This might cause conflicts, resolve them
git cherry-pick 7599162c
# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/52816/stable31

Error: Failed to push branch backport/52816/stable31: remote: {"auth_status":"auth_error","body":"Invalid username or token. Password authentication is not supported for Git operations."}
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.