[python] Set soma_joinid to always be an index column in the PointCloudDataFrame class

[jp-dark]

Issue and/or context: sc-61690

Changes:
This changes the soma_joinid to be an index_column in the PointCloudDataFrame. The initial implementation allowed soma_joinid to be an optional column. It was then dropped. Work on querying between observations/measurements and spatial data has motivated adding it back to improve querying by obs_id and var_id.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.27%. Comparing base (7616147) to head (4dd59d6).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3563      +/-   ##
==========================================
+ Coverage   86.22%   86.27%   +0.04%     
==========================================
  Files          55       55              
  Lines        6410     6410              
==========================================
+ Hits         5527     5530       +3     
+ Misses        883      880       -3     

Flag	Coverage Δ
python	86.27% <50.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	86.27% <50.00%> (+0.04%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[github-actions]

The backport to release-1.15 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-1.15 release-1.15
# Navigate to the new working tree
cd .worktrees/backport-release-1.15
# Create a new branch
git switch --create backport-3563-to-release-1.15
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 ec90ec7108b8b5943011fb256b4e1e4b9fd68952
# Push it to GitHub
git push --set-upstream origin backport-3563-to-release-1.15
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-1.15

Then, create a pull request where the base branch is release-1.15 and the compare/head branch is backport-3563-to-release-1.15.