feat: add Close method to BaseClient

[MattDevy]

• Adds Close method to BaseClient
• Fixes Bulk Indexer Close behaviour
• Documents new Close method

Relates to: #875

[prodsecmachine]

✅ Snyk checks have passed. No issues have been found so far.

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✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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[github-actions]

🔍 Preview links for changed docs

• docs/reference/connecting.md
• docs/reference/esql.md
• docs/reference/index.md
• docs/reference/installation.md

[elastic-vault-github-plugin-prod]

The backport to 8.19 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-8.19 8.19
# Navigate to the new working tree
cd .worktrees/backport-8.19
# Create a new branch
git switch --create backport-1056-to-8.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9d9d1911ef368906e09dd872ae641620eb9d3d53
# Push it to GitHub
git push --set-upstream origin backport-1056-to-8.19
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.19

Then, create a pull request where the base branch is 8.19 and the compare/head branch is backport-1056-to-8.19.