Define and use a get_log_add_schema() function

[scovich]

We have a convenient get_log_schema() function for accessing the Delta log schema, but many operations only need to access the add column. Replace clunky projection code with something direct and infallible, wrapped up as get_log_add_schema().

While we're at it, make both functions return SchemaRef instead of Schema, to reduce the number of deep clones needed (many use sites anyway need an arc).

[zachschuermann]

just an overall question: do we want the static LOG_*SCHEMA to be an Arc? doesn't a static reference also implement the Send/Sync/etc. that we need? (and possibly marginally lighter weight?)

[scovich]

just an overall question: do we want the static LOG_*SCHEMA to be an Arc? doesn't a static reference also implement the Send/Sync/etc. that we need? (and possibly marginally lighter weight?)

Rationale (from PR description):

make both functions return SchemaRef instead of Schema, to reduce the number of deep clones needed (many use sites anyway need an arc).

Basically, it's easy/cheap to get a &Schema from an arc.. not so cheap to get an arc from a borrowed reference.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.82%. Comparing base (8de8689) to head (eb3a7e0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/actions/mod.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #407      +/-   ##
==========================================
+ Coverage   78.80%   78.82%   +0.02%     
==========================================
  Files          51       51              
  Lines       10647    10655       +8     
  Branches    10647    10655       +8     
==========================================
+ Hits         8390     8399       +9     
  Misses       1788     1788              
+ Partials      469      468       -1     

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[zachschuermann]

LGTM

[nicklan]

lgtm, thanks!