Support configuring the config server URL

[rtuck99]

This enables the config server URL to be specified in the external callbacks, which allows system tests to be fixed.

Requires:

• Configure the config server via an environment variable dodal#2008

Adds additional documentation for environment variables.
The external callbacks must now be launched with the CONFIG_SERVER_URL parameter specified in order to start

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.88%. Comparing base (99eb139) to head (abea743).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1698   +/-   ##
=======================================
  Coverage   92.88%   92.88%           
=======================================
  Files         155      155           
  Lines        8500     8509    +9     
=======================================
+ Hits         7895     7904    +9     
  Misses        605      605           

Components	Coverage Δ
i24 SSX	77.35% <ø> (ø)
hyperion	98.59% <100.00%> (+<0.01%)	⬆️
other	98.31% <ø> (ø)

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