Support inline agent for agent execution

[Hailong-am]

Description

An example usage:

POST /_plugins/_ml/agents/_execute
{
  "agent": {
    "name": "List cluster index",
    "type": "flow",
    "tools": [
      {
        "name": "List cluster index",
        "type": "ListIndexTool",
        "parameters": {}
      }
    ]
  },
  "parameters": {
    "index": "ss4o*"
  }
}

Related Issues

#3998

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[Hailong-am]

Build pass locally

./gradlew build 
=======================================
OpenSearch Build Hamster says Hello!
  Gradle Version        : 8.11.1
  OS Info               : Linux 5.10.239-216.958.amzn2int.x86_64 (amd64)
  JDK Version           : 21 (Amazon Corretto JDK)
  JAVA_HOME             : /usr/lib/jvm/java-21-amazon-corretto
  Random Testing Seed   : 8DF3B361B927C4A4
  Crypto Standard       : any-supported
=======================================

BUILD SUCCESSFUL in 28m 45s
102 actionable tasks: 7 executed, 95 up-to-date