Fixing MachineLearningNodeClient create connector, deploy model, register model group actions

[joshpalis]

Description

Following PR #1560, additional class casting issues have been observed for create connector, deploy model, register model group.

This PR resolves those issues

Issues Resolved

Coming from opensearch-project/flow-framework#131
Continuation of #1553

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[joshpalis]

@ylwu-amzn This PR, along with the following PRs need to be backported to the agent_framework feature branch. Flow framework plugin 2.x will be tracking ML-Commons agent_framework instead of 2.x as per this issue. Can you assist in creating and applying the backport label

PRs :

• Added create connector API for MLClient #1437
• Added register model group API for MLClient #1493
• fix register client API #1560
• add register and deploy api in client (#1359) #1388

[opensearch-trigger-bot]

The backport to feature/agent_framework_dev 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-feature/agent_framework_dev feature/agent_framework_dev
# Navigate to the new working tree
cd .worktrees/backport-feature/agent_framework_dev
# Create a new branch
git switch --create backport/backport-1580-to-feature/agent_framework_dev
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 882246c04daacc98eb9c72fbdcabcce497856423
# Push it to GitHub
git push --set-upstream origin backport/backport-1580-to-feature/agent_framework_dev
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-feature/agent_framework_dev

Then, create a pull request where the base branch is feature/agent_framework_dev and the compare/head branch is backport/backport-1580-to-feature/agent_framework_dev.