New command for injecting new metadata in an EPUB

[HadrienGardeur]

As part of our work to prepare for EAA, we need to work on a new rwp command that supports injecting new metadata in an EPUB package.

Some of the typical use cases will include:

• Injecting TDM Rep metadata
• Adding an accessibility summary for EPUB files that haven't been remediated yet
• Injecting metadata post-remediation and/or based on trusted inferences

For now, we'll keep this simple by only allowing new metadata to be injected, in order to avoid conflicts. We'll also limit this initial support to EPUB 3.x files for now.