[SBOM] Add an API to retrieve the SBOM content for the SBOM accessory

[stonezdj]

Add an API to retrieve the SBOM content for the SBOM accessory
Request Method:

GET /api/v2.0/projects/<projectname>/repositories/<repository>/artifacts/<digest>/additions/sbom

Response:

{
    "spdxVersion": "SPDX-2.3",
    "dataLicense": "CC0-1.0",
    "SPDXID": "SPDXRef-DOCUMENT",
    "name": "alpine:latest",
    "documentNamespace": "<http://aquasecurity.github.io/trivy/container_image/alpine:latest-24fab3cb-05fa-479b-b3b7-f76151354cc3>",
    "creationInfo":{
      "licenseListVersion": "",
      "creators": [
        "Organization: aquasecurity",
        "Tool: trivy-0.44.1"
      ],
      "created": "2023-09-22T07:41:04Z"
    },
    "packages": [
        {
          "name": "alpine",
          "SPDXID": "SPDXRef-OperatingSystem-68bf9b9d283c287a",
          "versionInfo": "3.18.3",
          "downloadLocation": "NONE",
          "copyrightText": "",
          "primaryPackagePurpose": "OPERATING-SYSTEM"
        },
        ...
    ]
    ...
    }
   ...
}