Transfers: Refactor activity priority plugin into ActivityBasedTransferPriorityPlugin, and other refactors

[rdimaio]

Fixes #7137

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 1.38%. Comparing base (192a2ac) to head (7e8e1bc). Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/rucio/transfertool/fts3_plugins.py	0.00%	19 Missing :warning:

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #7144   +/-   ##
======================================
  Coverage    1.38%   1.38%           
======================================
  Files         265     265           
  Lines       41654   41654           
======================================
  Hits          576     576           
  Misses      41078   41078           

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[rdimaio]

Rebased in latest revision