[rom_ext] Add mechanism to invalidate ROM_EXT keys

Nobody has claimed this yet.

### Description

The ROM_EXT needs a mechanism to disable verification keys using a similar to what is supported in the ROM. However, the ROM_EXT mechanism needs to be able to target the specific key stored in the ROM_EXT binary when Ownership Transfer is not available. This means we cannot rely just on the key index with respect to the key array (as done in the ROM), because this is subject to change based on the ROM_EXT configuration. For example, there may be two different ROM_EXT key configurations to provide a product with two different SKUs.

## Requirements
* SKUs need to support different ROM_EXT key domains to fulfill potential downstream integration requirements. Each Silicon Owner gets a set of keys provisioned in the ROM_EXT.
* The key invalidation mechanism needs to be specific for the key domain deployed in a particular ROM_EXT configuration.

## Opens
1. Does the ROM_EXT need to remember any previously invalidated keys? For example,
1.1. There may be a new ROM_EXT for any particular SKU that rotates all they keys with respect to the previous binary.
The new version of the ROM_EXT is anti rollback protected by the ROM.
1.2. There may be a new ROM_EXT binary to effectively "transfer ownership" of the device to a new owner. In this case
the device is assumed not to have any secrets stored by the previous owner.

Start by comparing the existing ROM key-invalidation mechanism with the ROM_EXT requirements, especially targeting keys by domain rather than array index. Resolve whether invalidated keys must persist across rotated or ownership-transferred ROM_EXT binaries, then define the mechanism and its completion criteria; the issue names no files or tests to run.

Written by the indexing model from the issue text.