Add ServiceRegistry to help migration

[a1q123456]

High Level Overview of Change

This PR adds a service registry class.

Context of Change

We're passing Application around and the Application class acts more like a service registry that gives other classes access to other services. In order to allow modularisation, we should replace Application with a service registry class so that modules depending on Application for other services can be moved easily.

This PR is part of the modularisation of the transactors. See all PRs here:
#6222
#6223
#6224
#6225
#6226
#6227
#6228

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.3%. Comparing base (c9458b7) to head (ea60898).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6222   +/-   ##
=======================================
  Coverage     79.3%   79.3%           
=======================================
  Files          839     842    +3     
  Lines        71609   71691   +82     
  Branches      8286    8282    -4     
=======================================
+ Hits         56774   56864   +90     
+ Misses       14835   14827    -8     

Files with missing lines	Coverage Δ
include/xrpl/core/ServiceRegistry.h	100.0% <100.0%> (ø)
src/xrpld/app/main/Application.cpp	70.1% <100.0%> (+1.5%)	⬆️
src/xrpld/app/main/Application.h	100.0% <ø> (ø)
src/xrpld/core/ServiceRegistryImpl.h	100.0% <100.0%> (ø)
src/xrpld/core/detail/ServiceRegistryImpl.cpp	100.0% <100.0%> (ø)

... and 6 files with indirect coverage changes

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