feat: package db diff for v6

[kzantow]

This PR adds a db diff command that implements package-based diffing between two Grype v6 databases.

The grype db diff <db1> <db2> command in this PR operates by comparing two grype databases from the perspective of "what changes have affected matching of specific packages" -- in other words: any data change that will result in a vulnerability being returned differently for a specific package should be returned as either an addition, modification, or removal. For example: if a vulnerability is added, it should would expectedly show up in the added section, whereas if a vulnerability has had the version constraints modified, it would be considered modified, and similarly if a vulnerability for a specific package in a specific ecosystem / provider has been removed, it would show up in the removed section.

The output is currently defined in the db-diff schema, for example:

{
  "schema": "anchore.io/schema/grype/db-diff/json/0.5.0/results",
  "databases": {
    "before": {
      "buildTimestamp": "2026-02-07T06:24:08Z",
      "modelVersion": "v6.1.4",
      "revision": 1,
      "checksum": "xxh64:a2fac5f546c7b9b7"
    },
    "after": {
      "buildTimestamp": "2026-02-09T06:37:39Z",
      "modelVersion": "v6.1.4",
      "revision": 1,
      "checksum": "xxh64:3ea413099fbdbe37"
    }
  },
  "packages": [
    {
      "ecosystem": "apk",
      "name": "cloudflared-fips",
      "vulnerabilities": {
        "added": [
          {
            "provider": "chainguard",
            "id": "CVE-2025-58063"
          },
        ]
      }
    },
    {
      "ecosystem": "apk",
      "name": "glibc-2.41",
      "vulnerabilities": {
        "modified": [
          {
            "provider": "secureos",
            "id": "CVE-2025-5702"
          },
...

In order to get reasonable performance , a series of temp tables and views are created in the "old" database. These can be written to the database directly using the environment variable GRYPE_DEV_DB_DEBUG=true for further investigation about the results.