Add support for group members to have names

[Shelnutt2]

This adjusts the group API to allow members to have an optional name and introduces a new tiledb_group_get_member_by_name function. Names are helpful to have ways to add specific members but reference them directly by a given name. Names must be unique in the group. The name is optional, if a nullptr is based in then the name is not stored and the group member must be accessed through the index methods. The C++ API handles this with std::optional.

This request came in from @jp-dark as part of the group API review for use in applications like netcdf. I'm tagging a larger-than normal reviewer list for awareness as this will be a "breaking" change in the unreleased API (compared to existing RCs). This was briefly discussed with @aaronwolen and @stavrospapadopoulos before making the change. In general everyone agreed on the usefulness so we're proposing to add it to the initial groups in 2.8. The overall change set is small, but of course Cap'n Proto was updated so thats majority of the line number changes.

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TYPE: IMPROVEMENT
DESC: Add ability to store (optional) name with group member

[shortcut-integration]

This pull request has been linked to Shortcut Story #16795: Add support for name/alias to group members.

[github-actions]

The backport to release-2.8 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-2.8 release-2.8
# Navigate to the new working tree
cd .worktrees/backport-release-2.8
# Create a new branch
git switch --create backport-3068-to-release-2.8
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 87905fbe0666bea5e9ef1dbcf86604a26586bd28
# Push it to GitHub
git push --set-upstream origin backport-3068-to-release-2.8
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-2.8

Then, create a pull request where the base branch is release-2.8 and the compare/head branch is backport-3068-to-release-2.8.