feat: Introduce readme element to Kraftfile

[nderjung]

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran make fmt on your commit series before opening this PR;
• Updated relevant documentation.

Description of changes

This PR introduces the ability to specify either a string or path to a README text file in a Kraftfile. At the same time, this is propagated to the OCI index as an annotation. In the OCI annotation, the string contents is base64 encoded.

Example usage

1. As a short-form string:

   spec: v0.6
   readme: "This unikernel is amazing"

2. As a long-form string:

   spec: v0.6
   readme: |
     This unikernel is amazing!

3. As an external file:

   spec: v0.6
   readme: ./README.md

TODO

This PR is still in draft form, the following additional pieces of work need to occur:

• Propagate the package's readme into kraft pkg info

[craciunoiuc]

@nderjung how's this going?

[nderjung]

Not sure why it was converted out of draft state. There's still an outstanding TODO w.r.t. getting back this information, such that it appears in kraft pkg info.