feat(rest): add declaredLicense field to Release model

[krrish175-byte]

[[/]: # (This program and the accompanying materials are made)
[[/]: # (available under the terms of the Eclipse Public License 2.0)
[[/]: # (which is available at https://www.eclipse.org/legal/epl-2.0/)
[[/]: # (SPDX-License-Identifier: EPL-2.0)

Please provide a summary of your changes here.

Added a dedicated declaredLicense property to the Release Thrift struct to natively separate SPDX license expressions from standard associated licenses. It handles seamless JSON serialization of the new field and ensures existing client workflows that use the "declared_license" key in additionalData are gracefully supported via backwards-compatible getters.

• Which issue is this pull request belonging to and how is it solving it? (Refer to issue here)

This PR resolves #811 by providing a first-class property for genuine SPDX license expressions alongside mainLicenseIds and otherLicenseIds.

• Did you add or update any new dependencies that are required for your change?

No new dependencies were added.

Suggest Reviewer

You can suggest reviewers here with an @mention.

@mcjaeger @AlbertHjy

How To Test?

How should these changes be tested by the reviewer?

Ensure the standard build command works locally with Thrift generation. Fetch a release through the REST API and verify that declaredLicense is successfully populated in the JSON payload if it is set.

Have you implemented any additional tests?

Yes, added testing and documentation of the declaredLicense field to API assertions in ReleaseSpecTest.java.

Checklist

Must:

• All related issues are referenced in commit messages and in PR