Add description, homepage, documentation and repository fields for versions

[Turbo87]

We currently save these fields only per crate, not per version. This means that for crates publishing bug fixes to older major versions, these fields might get overwritten with outdated metadata.

The solution to this is to read these per crate metadata fields from the "default version" of a crate instead.

This PR introduces four more metadata columns to the versions table (description, homepage, documentation and repository) and exposes their values on the API. The rest of the code is not using these values yet since they will need to be backfilled from the stored crate files first. Once that has happened we could remove the fields from the crates table and use the content from the default version of a crate instead.

Potential downsides of this:

• database size: since we are storing mostly similar data for each version this might grow the database size considerably.
• database dump size: same as above, though compression should hopefully get rid of most of the overhead.
• database dump breaking change: if we remove the existing columns of the crates table, or if we stop updating them, this will essentially be a breaking change to the database dump schema. while we should not see this as a hard blocker, we should be careful to not introduce breaking changes too often.
• more joins in queries needed: this might have a performance impact, but with the existing indexes I expect this to be manageable, and we're already pulling in the default version for the relevant endpoint anyway, so the additional overhead might actually be zero.

Related:

• Display most recently published version next to "last updated" time on crate show page #953
• keywords and description seems to come from the last published version not actual version. #7560

[Turbo87]

RE database dump breaking change: I guess we could also denormalize the data from the versions to the crates table whenever we update the default version 🤔

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.50%. Comparing base (40bb88f) to head (1da5646).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9998      +/-   ##
==========================================
+ Coverage   89.49%   89.50%   +0.01%     
==========================================
  Files         295      295              
  Lines       31233    31257      +24     
==========================================
+ Hits        27951    27976      +25     
+ Misses       3282     3281       -1     

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[Turbo87]

7 days without any objections and we talked about this PR in the crates.io team meeting last week, also without any objections, so let's merge this :)