FR: Let the user customise the _PLATFORM_TO_BAZEL_CPU mapping and make the oci_pull platform logic stricter

The current behaviour of `oci_pull` for multi-arch images is that `platforms` entries absent from the `_PLATFORM_TO_BAZEL_CPU` mapping are silently omitted from the `select( ... )` in the `oci_pull` alias.

A better behavior would be to fail if *any* of the elements in the `platforms` attribute is not supported.

In order to ensure that users with exotic platforms don't get stuck with an impossible-to-fix error, we should also let the user specify their own platform-to-bazel-cpu mapping in case they need to handle platforms that are not currently present in the hardcoded `_PLATFORM_TO_BAZEL_CPU` mapping provided by `rules_oci`.

Start by tracing the oci_pull handling of the platforms attribute, the _PLATFORM_TO_BAZEL_CPU mapping, and the select(...) used in the alias. Verify how unsupported entries are currently omitted; done means users can provide an additional mapping and any unsupported platform causes a clear failure rather than being silently dropped.

Written by the indexing model from the issue text.