generate fig autocompletion

**What would you like to be added**:

**Why is this needed**:

We (w/@srenatus @anderseknert) did a [similar effort](https://github.com/withfig/autocomplete/pull/854) in OPA to add autocompletion of opa CLI in by [fig](https://github.com/withfig/autocomplete). So, let's do this for cosign too.


**Additional context**:

Start by reviewing the linked OPA autocomplete effort and the Fig autocomplete project, then compare that approach with the cosign CLI. The issue is complete when cosign has usable Fig autocompletion, but it does not name files, commands, or tests to guide implementation.

Written by the indexing model from the issue text.