Add joint conversion for fixed, continuous, revolute, prismatic, and ball types

[azeey]

🎉 New feature

Summary

Additional joint types, such as revolute2 and universal can be implemented by using multiple joints, but I think we can leave them for a follow up PR. The remaining joint types (gearbox and screw) will also handled in a later PR.

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #4 (20dd572) into main (0742043) will increase coverage by 3.37%.
The diff coverage is 90.27%.

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   83.33%   86.71%   +3.37%     
==========================================
  Files           4        5       +1     
  Lines          72      143      +71     
==========================================
+ Hits           60      124      +64     
- Misses         12       19       +7     

Impacted Files	Coverage Δ
sdformat_mjcf/sdformat_mjcf/sdf_utils.py	52.17% <28.57%> (-12.54%)	⬇️
sdformat_mjcf/sdformat_mjcf/converters/joint.py	96.92% <96.92%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0742043...20dd572. Read the comment docs.

[azeey]

Created issues #23 and #24 to track support for other joint types.