Add Target object for external data source support on Monitor and Alert models.

[manaswini1920]

Description

Add Target object for external data source support on Monitor and Alert models.

• New Target data class in common-utils with type (enum: LOCAL, AOSS_COLLECTION, AOS_DOMAIN) and endpoint (URL of remote collection/domain)
• Monitor: added target: Target field (defaults to {type: LOCAL})
• Alert: added target: Target? field, copied from parent Monitor at alert creation time across all 5 Monitor-taking constructors
• All new fields are optional with defaults for backward compatibility
• Target only serialized when non-LOCAL (conditional toXContent)
• StreamInput/writeTo version-gated to V_3_6_0 for rolling upgrade safety
• Includes TargetTests with 7 unit tests covering serialization, parsing, validation, and enum handling

Related Issues

Resolves

Testing

• 7 new unit tests in TargetTests — default construction, AOSS/AOS targets, endpoint validation, stream roundtrip, XContent roundtrip, enum parsing
• All existing tests pass — backward compatibility verified

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.