refactor: Make Metadata fields private

[lbhm]

What changes are proposed in this pull request?

This PR resolves #1296 and refactors the fields of Metadata to make them private.

The change forces all callers to instantiate Metadata via the try_new function. Otherwise, we cannot guarantee that metadata columns never leak to the Delta log, as callers might directly instantiate a Metadata instance with an illegal table schema.

This PR affects the following public APIs

None. All Metadata fields have been pub(crate).

How was this change tested?

This is a pure refactoring. Existing tests verify correctness.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.76%. Comparing base (a27f366) to head (7aa51d0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1347      +/-   ##
==========================================
+ Coverage   84.73%   84.76%   +0.02%     
==========================================
  Files         113      113              
  Lines       28396    28421      +25     
  Branches    28396    28421      +25     
==========================================
+ Hits        24062    24091      +29     
+ Misses       3198     3194       -4     
  Partials     1136     1136              

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[zachschuermann]

looks good! (perhaps we could take some follow up to add some testing utils so we have less verbose callsites but let's take that up later)