add cron task to fetch manifests

[tlambert03]

This will run a cron job twice a day to gather manifests and put them in the manifests branch

I actually don't expect this to work until it's on the main branch, so will likely merge to test. I tested on my personal branch and it works. It doesn't really affect any of the repo (except the addition of an --all flag to npe2 fetch)

[codecov]

Codecov Report

Merging #216 (f8e03b5) into main (f9ef486) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #216   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines         2230      2251   +21     
=========================================
+ Hits          2230      2251   +21     

Impacted Files	Coverage Δ
npe2/_fetch.py	100.00% <100.00%> (ø)
npe2/cli.py	100.00% <100.00%> (ø)

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