Enable the fetching of only the OCI descriptor

Some OCI artifacts need to have a specific media type (i.e. they should
only be an OCI image index or an OCI image manifest). In order to
determine that with a single remote call, we can just get the descriptor
and then check the mediaType of that object.

Signed-off-by: arewm <arewm@users.noreply.github.com>

docs/modules/ROOT/pages/ec_oci_descriptor.adoc

@@ -0,0 +1,36 @@
+= ec.oci.descriptor
+
+Fetch a raw Image from an OCI registry.
+
+== Usage
+
+  object = ec.oci.descriptor(ref: string)
+
+== Parameters
+
+* `ref` (`string`): OCI descriptor reference
+
+== Return
+
+`object` (`object`): the OCI descriptor object
+
+The object contains the following attributes:
+
+* `annotations` (`object`)
+** (`string`): (`string`)
+* `artifactType` (`string`)
+* `data` (`string`)
+* `digest` (`string`)
+* `mediaType` (`string`)
+* `platform` (`object`)
+** `architecture` (`string`)
+** `features`(`array`)
+*** (`string`)
+** `os` (`string`)
+** `os.features`(`array`)
+*** (`string`)
+** `os.version` (`string`)
+** `variant` (`string`)
+* `size` (`number`)
+* `urls`(`array`)
+** (`string`)

docs/modules/ROOT/pages/rego_builtins.adoc

@@ -10,6 +10,8 @@ information.
 |===
 |xref:ec_oci_blob.adoc[ec.oci.blob]
 |Fetch a blob from an OCI registry.
+|xref:ec_oci_descriptor.adoc[ec.oci.descriptor]
+|Fetch a raw Image from an OCI registry.
 |xref:ec_oci_image_files.adoc[ec.oci.image_files]
 |Fetch structured files (YAML or JSON) from within an image.
 |xref:ec_oci_image_manifest.adoc[ec.oci.image_manifest]

docs/modules/ROOT/partials/rego_nav.adoc

@@ -1,5 +1,6 @@
 * xref:rego_builtins.adoc[Rego Reference]
 ** xref:ec_oci_blob.adoc[ec.oci.blob]
+** xref:ec_oci_descriptor.adoc[ec.oci.descriptor]
 ** xref:ec_oci_image_files.adoc[ec.oci.image_files]
 ** xref:ec_oci_image_manifest.adoc[ec.oci.image_manifest]
 ** xref:ec_purl_is_valid.adoc[ec.purl.is_valid]