bug: fix missing mountpoints in COSI

[elainezhao1]

PR summary:

This is a fix for missing mountpoints in COSI metadata. Created partuuid to mountpath mapping, and depending on whether we do partition customization or not, we use the mapping created during partition customization or the mapping in the existing image.

local test:
now we are able to see the mountpoints information in COSI metadata.

tested with command sudo ./imagecustomizer --log-level debug --build-dir ./build --image-file core-3.0.20241220.vhdx --output-image-file ./output-image-test.cosi --output-image-format cosi --config-file ../pkg/imagecustomizerlib/testdata/verity-config.yaml

{
  "version": "1.0",
  "osArch": "amd64",
  "images": [
    {
      "image": {
        "path": "images/output-image-test_1.raw.zst",
        "compressedSize": 988959,
        "uncompressedSize": 8388608,
        "sha384": "40bc37dc53dff6194bc35b803b824b5a94f42e99235d50944125bf899bffc7e43aa7f815bfea86e1b4d0353efa11142c"
      },
      "mountPoint": "/boot/efi",
      "fsType": "vfat",
      "fsUuid": "F927-3DD3",
      "partType": "c12a7328-f81f-11d2-ba4b-00a0c93ec93b",
      "verity": null
    },
    {
      "image": {
        "path": "images/output-image-test_2.raw.zst",
        "compressedSize": 51438315,
        "uncompressedSize": 1064304640,
        "sha384": "d7e0cf15b8a13bcfd7f84345dff1e8b3769d3cb2b772a83904a61be7d091cf7da2202a3d6ee3e85a709bb0ce636151df"
      },
      "mountPoint": "/boot",
      "fsType": "ext4",
      "fsUuid": "ff2999cd-c34a-4e12-a93c-f8a97a55fbe9",
      "partType": "0fc63daf-8483-4772-8e79-3d69d8477de4",
      "verity": null
    },
    {
      "image": {
        "path": "images/output-image-test_3.raw.zst",
        "compressedSize": 114697262,
        "uncompressedSize": 2147483648,
        "sha384": "783fe87af97ac5f5ea683c1887e46d58029ee31a15c4439757337dbf5118fb1d2a95d22bd5c83fd84d6fb5936bf6ce57"
      },
      "mountPoint": "/",
      "fsType": "ext4",
      "fsUuid": "8052a630-d72d-420e-a0f5-62ce6e04eb27",
      "partType": "0fc63daf-8483-4772-8e79-3d69d8477de4",
      "verity": {
        "image": {
          "path": "images/output-image-test_4.raw.zst",
          "compressedSize": 2894248,
          "uncompressedSize": 134217728,
          "sha384": "1b0e2dc39598a5fb8b4cfaa7aa2698c0806190ae562480544ec4b82f2e182b3fa118e0c8e923f032cb7aecdc9325ce18"
        },
        "hash": "6271f33e97b7d8853b5fc685c35cdd200d9b9a0b911d4fe79e50294800ba8cd6"
      }
    },
    {
      "image": {
        "path": "images/output-image-test_5.raw.zst",
        "compressedSize": 28788772,
        "uncompressedSize": 2012217344,
        "sha384": "1452e87e66370956664b2228de28ddb3cc0f8ca0ebc91f6f555a08c61bcf9e660533d9a73c1e5a1b778c22f33ad771a8"
      },
      "mountPoint": "/var",
      "fsType": "ext4",
      "fsUuid": "960b53c3-2023-40e0-8388-0f07c31952c0",
      "partType": "0fc63daf-8483-4772-8e79-3d69d8477de4",
      "verity": null
    }
  ],
  "osRelease": "",
  "id": "601d3fac-e656-20f0-ef51-cd08c591bd84"
}

without partition customization,
tested with command sudo ./imagecustomizer --log-level debug --build-dir ./build --image-file core-3.0.20241220.vhdx --output-image-file ./output-image-test.cosi --output-image-format cosi --config-file ../pkg/imagecustomizerlib/testdata/users-config.yaml

{
  "version": "1.0",
  "osArch": "amd64",
  "images": [
    {
      "image": {
        "path": "images/output-image-test_1.raw.zst",
        "compressedSize": 986231,
        "uncompressedSize": 8388608,
        "sha384": "7d59ebf20375104232adffdd502f4f6e1044982e26c20c1153f7d74f407412549cdd183c9540a0c72657d21104c19fad"
      },
      "mountPoint": "/boot/efi",
      "fsType": "vfat",
      "fsUuid": "2BC1-1CF9",
      "partType": "c12a7328-f81f-11d2-ba4b-00a0c93ec93b",
      "verity": null
    },
    {
      "image": {
        "path": "images/output-image-test_2.raw.zst",
        "compressedSize": 190945452,
        "uncompressedSize": 543162368,
        "sha384": "019f5ed59258da9b9265bc77473d5ab9853c172c8ae77aaaf8a3569f91f69d601bf116a3db875e67b7d5438365e5bba6"
      },
      "mountPoint": "/",
      "fsType": "ext4",
      "fsUuid": "b8fe0f63-73ee-433f-858a-75fd3e762c79",
      "partType": "0fc63daf-8483-4772-8e79-3d69d8477de4",
      "verity": null
    }
  ],
  "osRelease": "",
  "id": "70121e96-3c8b-d76c-f3e0-eeda75fcb776"

---

Checklist

• Tests added/updated
• Documentation updated (if needed)
• Code conforms to style guidelines

[cwize1]

nit: Move duplicate code to a separate function.