Fix manifest annotations

[flavio]

The main change introduced by this PR is to better support OCI annotations:

• Define a series of constants, one per pre-defined OCI annotation. This one of our TODOs
• Extend Config and Manifest and other objects to allow them to have OCI annotations, as expressed by the OCI Image Spec
• Provide an helper constructor for OciImageIndex. That allows users to create a Manifest with annotations, that can then be passed to the Client::push method. This helper is obtained by converting a private helper function into a public one
• Do not set a default org.opencontainers.image.title annotation inside of the ImageLayers. This fixes issue #30
• Extend the example wasm push command to make use of these changes

This PR introduces 2 small changes that are unrelated with the main topic:

• Fix links inside of inline docs: address the cargo doc warnins. A lot of our inline links were not rendered as hyperlinks, but as simple text
• Small refactoring of some existing code: make use of an internal helper function to remove some duplicated code

Note: this PR builds on top of https://github.com/krustlet/oci-distribution/pull/33 I'll rebase it gets merged

[flavio]

Changes done, I'll rebase the code once #33 is merged

[flavio]

rebased