feat: Add AppContextWeak to prevent reference cycles

[spencewenski]

This is useful for preventing reference cycles between things that are held in the AppContext and also need a reference to the AppContext; for example, HealthChecks.

Closes #526

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 40.54%. Comparing base (ef4508b) to head (04a7260).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/health_check/mod.rs	0.00%	11 Missing ⚠️
src/health_check/sidekiq_fetch.rs	28.57%	10 Missing ⚠️
src/health_check/database.rs	44.44%	5 Missing ⚠️
src/health_check/email/smtp.rs	44.44%	5 Missing ⚠️
src/health_check/sidekiq_enqueue.rs	44.44%	5 Missing ⚠️

Files with missing lines	Coverage Δ
src/app/context.rs	14.33% <100.00%> (+2.52%)	⬆️
src/health_check/default.rs	100.00% <100.00%> (ø)
src/health_check/database.rs	70.83% <44.44%> (-11.52%)	⬇️
src/health_check/email/smtp.rs	63.15% <44.44%> (-11.85%)	⬇️
src/health_check/sidekiq_enqueue.rs	70.83% <44.44%> (-11.52%)	⬇️
src/health_check/sidekiq_fetch.rs	60.00% <28.57%> (-2.50%)	⬇️
src/health_check/mod.rs	4.76% <0.00%> (-5.24%)	⬇️

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