chore: unify and improve message context

[zepatrik]

1. All context keys should be snake_case and not camelCase. Wrongly formatted keys are kept to not break integrations.
2. Some messages that had no context had the empty object as the payload, others omitted the key completely. For consistency, the key is now always omitted when the message has no context.
3. Documentation was aligned with the context keys.

[codecov]

Codecov Report

Merging #3462 (d02708a) into master (eaaf375) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 96.72%.

❗ Current head d02708a differs from pull request most recent head 546e971. Consider uploading reports for the commit 546e971 to get more accurate results

@@           Coverage Diff           @@
##           master    #3462   +/-   ##
=======================================
  Coverage   78.23%   78.23%           
=======================================
  Files         339      339           
  Lines       22495    22466   -29     
=======================================
- Hits        17598    17577   -21     
+ Misses       3584     3579    -5     
+ Partials     1313     1310    -3     

Files Changed	Coverage Δ
text/context.go	42.85% <50.00%> (-28.58%)	⬇️
text/message_validation.go	97.84% <92.30%> (-2.16%)	⬇️
cmd/clidoc/main.go	70.88% <100.00%> (ø)
schema/errors.go	90.87% <100.00%> (+3.75%)	⬆️
text/message_login.go	100.00% <100.00%> (ø)
text/message_node.go	100.00% <100.00%> (ø)
text/message_recovery.go	100.00% <100.00%> (ø)
text/message_registration.go	100.00% <100.00%> (ø)
text/message_settings.go	100.00% <100.00%> (ø)
text/message_system.go	100.00% <100.00%> (ø)
... and 1 more

... and 1 file with indirect coverage changes