Fix PutIndexTemplateRequest deserialization

[matthewbogner]

Description

This PR adds in the missing deserialization steps to the PutIndexTemplateRequest _DESERIALIZER which would result in the error
Missing required property 'PutIndexTemplateRequest.name' if you tried to deserialize a request object.

Issues Resolved

Relates to issues #417, #618 and PR #723

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

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-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-765-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 d4aade4e866556f8a11ed05756ab8b01f2ed0e71
# Push it to GitHub
git push --set-upstream origin backport/backport-765-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-765-to-2.x.

[dblock]

@matthewbogner Looks like this will need a manual backport if you want the fix in a 2.x release.