Add two new configuration options for periodic retained WAL size check in scaled down mode

[alco]

Closes https://github.com/electric-sql/electric/issues/3260.

This PR is missing the query for WAL size but the surrounding logic is ready for review.

I'll implement the query in a follow-up PR because I want to build a basic synchronous interface on top of Postgrex.SimpleConnection.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 16.66667% with 30 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 67.05%. Comparing base (60c8d10) to head (bd2bd19).

Files with missing lines	Patch %	Lines
.../sync-service/lib/electric/connection/restarter.ex	0.00%	20 Missing :warning:
...es/sync-service/lib/electric/connection/manager.ex	0.00%	9 Missing :warning:
packages/sync-service/lib/electric/config.ex	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3274      +/-   ##
==========================================
- Coverage   69.83%   67.05%   -2.79%     
==========================================
  Files         181      172       -9     
  Lines        9777     8872     -905     
  Branches      336      106     -230     
==========================================
- Hits         6828     5949     -879     
+ Misses       2947     2922      -25     
+ Partials        2        1       -1     

Flag	Coverage Δ
elixir	66.77% <16.66%> (-0.22%)	:arrow_down:
elixir-client	73.94% <ø> (ø)
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/typescript-client	?
packages/y-electric	55.12% <ø> (ø)
postgres-140000	65.81% <16.66%> (-0.18%)	:arrow_down:
postgres-150000	65.85% <16.66%> (-0.22%)	:arrow_down:
postgres-170000	65.78% <16.66%> (-0.15%)	:arrow_down:
postgres-180000	?
sync-service	66.06% <16.66%> (-0.24%)	:arrow_down:
typescript	72.39% <ø> (-14.86%)	:arrow_down:
unit-tests	67.05% <16.66%> (-2.79%)	:arrow_down:

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