Support cleaning up extra backup sites

[ettanany]

This adds a new setting called extra_backup_sites_prefixes, which when set, makes cleaning process look into those extra sites as well, and this is particularly useful when we have old backups from a previous backup site after a PG major version upgrade.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.76%. Comparing base (5505b86) to head (b9980a9). Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
- Coverage   91.01%   90.76%   -0.25%     
==========================================
  Files          31       31              
  Lines        4917     4980      +63     
==========================================
+ Hits         4475     4520      +45     
- Misses        442      460      +18     

Files	Coverage Δ
pghoard/pghoard.py	86.62% <100.00%> (+1.04%)	:arrow_up:

... and 9 files with indirect coverage changes