Add support for composable index templates

[Xtansia]

Description

Adds support for composable index templates

Issues Resolved

Closes #404

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[Xtansia]

@dblock When you have time could you review this please?

[dblock]

Did a cursory review, looks ok to me.

[dblock]

Open an issue and link it from here.

[dblock]

I think for a guide we may want to Console.WriteLine instead of Debug.Assert.

[Xtansia]

@dblock Have made your suggested changes

[opensearch-trigger-bot]

The backport to 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-1.x 1.x
# Navigate to the new working tree
cd .worktrees/backport-1.x
# Create a new branch
git switch --create backport/backport-437-to-1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 dc8612c9ff9beaf5f184e9e0066b04845bee7cf1
# Push it to GitHub
git push --set-upstream origin backport/backport-437-to-1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.x

Then, create a pull request where the base branch is 1.x and the compare/head branch is backport/backport-437-to-1.x.