generic fetcher: Official support with ADR

This change includes the generic fetcher ADR which is a necessary
step for the feature to be officially supported.

Signed-off-by: Jan Koscielniak <jakoscie@redhat.com>

docs/adr/0001-add-generic-fetcher.md

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+# Add generic fetcher
+
+## Context
+
+Some users need to download arbitrary files that don't fit within an established package ecosystem cachi2 could
+potentially otherwise support. The target audience is users that want to use cachi2 to achieve hermetic builds
+and also want an easy way to include these arbitrary files, that cachi2 will account for in the SBOM it produces.
+
+## Decision
+
+A new package manager for generic artifacts must be introduced. This package manager utilizes a custom
+lockfile based on which it will download files, save them into a requested location, and verify checksums.
+Below is a more detailed overview of the implementation.
+
+### Lockfile format
+
+Cachi2 expects the lockfile to be named `artifacts.lock.yaml`.
+In order to account for possible future breaking changes, the lockfile will contain a `metadata` section with a `version`
+field that will indicate the version of the lockfile format. It will also contain a list of artifacts (files) to download,
+each of the artifacts to having a URL, a checksum, and optionally output filename specified.
+
+```yaml
+metadata:
+  # uses X.Y semantic versioning
+  version: "1.0"
+artifacts:
+  - download_url: https://huggingface.co/instructlab/granite-7b-lab/resolve/main/model-00001-of-00003.safetensors?download=true
+    filename: granite-model-1.safetensors
+    checksum: sha256:d16bf783cb6670f7f692ad7d6885ab957c63cfc1b9649bc4a3ba1cfbdfd5230c
+```
+
+#### Lockfile properties
+
+Below is an explanation of individual properties of the lockfile.
+
+##### download_url (required)
+
+Specified as a string containing the download url of the artifact.
+
+##### checksum (required)
+
+Specified as string in the format of "algorithm:hash". Must be provided to ensure the identity of the artifact.
+
+#### filename (optional)
+
+This key is provided mainly for the users convenience, so the files end up in expected locations. It is optional and if
+not specified, it will be derived from the download_url. Filename here is a path inside cachi2's output directory for
+the generic fetcher (`{cachi2-output-dir}/deps/generic`). Cachi2 will verify that the resulting filenames, including those
+derived from download urls do not overlap.
+
+### SBOM components
+
+Artifacts fetched with the generic fetcher will all be recorded in the SBOM cachi2 produces. Given the inability to derive
+any extra information about these files beyond a download location and a filename, these files will always be recorded
+as SBOM components with purl of type generic.
+
+Additionally, the SBOM component will contain [externalReferences] of type `distribution` to indicate the url used to download
+the file to allow for easier handling for tools that might process the SBOM.
+
+Here's an example SBOM generated for above file.
+
+```json
+{
+  "bomFormat": "CycloneDX",
+  "components": [
+    {
+      "name": "granite-model-1.safetensors",
+      "purl": "pkg:generic/granite-model-1.safetensors?checksum=sha256:d16bf783cb6670f7f692ad7d6885ab957c63cfc1b9649bc4a3ba1cfbdfd5230c&download_url=https://huggingface.co/instructlab/granite-7b-lab/resolve/main/model-00001-of-00003.safetensors",
+      "properties": [
+        {
+          "name": "cachi2:found_by",
+          "value": "cachi2"
+        }
+      ],
+      "type": "file",
+      "externalReferences": [
+        {
+          "url": "https://huggingface.co/instructlab/granite-7b-lab/resolve/main/model-00001-of-00003.safetensors",
+          "type": "distribution"
+        }
+      ]
+    }
+  ],
+  "metadata": {
+    "tools": [
+      {
+        "vendor": "red hat",
+        "name": "cachi2"
+      }
+    ]
+  },
+  "specVersion": "1.4",
+  "version": 1
+}
+```
+
+## Consequences
+
+As mentioned before, this package manager enables users to fetch arbitrary files with cachi2 and have them accounted for
+in the SBOM.
+
+[externalReferences]: https://cyclonedx.org/docs/1.6/json/#components_items_externalReferences