feat: add volume format property

[phm07]

This property was previously missing, although it was already used in VolumeCreateOpts. It's now a string pointer with common values (ext4 and xfs) available as string constants.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.35%. Comparing base (49be506) to head (f22592a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #397   +/-   ##
=======================================
  Coverage   80.34%   80.35%           
=======================================
  Files          32       32           
  Lines        5729     5731    +2     
=======================================
+ Hits         4603     4605    +2     
  Misses        718      718           
  Partials      408      408           

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[github-actions]

The backport to release-1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-1.x release-1.x
# Navigate to the new working tree
cd .worktrees/backport-release-1.x
# Create a new branch
git switch --create backport-397-to-release-1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c0940afce9eb01c0e6838502c91aa569ab411a03
# Push it to GitHub
git push --set-upstream origin backport-397-to-release-1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-1.x

Then, create a pull request where the base branch is release-1.x and the compare/head branch is backport-397-to-release-1.x.

[apricote]

@phm07 Can you backport this?

[phm07]

@phm07 Can you backport this?

Done in #444