PG-2379 Allow different principal keys for relation key file migration

[AndersAstrand]

The code assumed that all databases would use the same principal key which is generally not true. This fix loads the principal key for each database as needed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-2.2.0@f27fc09). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-2.2.0     #581   +/-   ##
================================================
  Coverage                 ?   57.65%           
================================================
  Files                    ?       68           
  Lines                    ?    10776           
  Branches                 ?     2657           
================================================
  Hits                     ?     6213           
  Misses                   ?     3286           
  Partials                 ?     1277           

Components	Coverage Δ
access	81.80% <0.00%> (?)
bin	63.76% <0.00%> (?)
catalog	78.34% <0.00%> (?)
common	88.23% <0.00%> (?)
encryption	57.02% <0.00%> (?)
keyring	65.55% <0.00%> (?)
src	87.33% <0.00%> (?)
smgr	89.54% <0.00%> (?)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rjd15372]

LGTM