feat: add singleOnly to extension

[KaWaite]

Overview

Added new field singleOnly to plugin extensions so that we can limit extension use when an extension will break with more than one instance (ie. builtin Storytelling or Splashscreen widgets).

[codecov]

Codecov Report

Merging #60 (44cf32f) into main (579b7a5) will increase coverage by 0.00%.
The diff coverage is 18.91%.

@@           Coverage Diff           @@
##             main      #60   +/-   ##
=======================================
  Coverage   16.50%   16.51%           
=======================================
  Files         246      246           
  Lines       43023    43060   +37     
=======================================
+ Hits         7103     7110    +7     
- Misses      34913    34943   +30     
  Partials     1007     1007           

Impacted Files	Coverage Δ
internal/adapter/gql/generated.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/mongodoc/plugin.go	0.00% <0.00%> (ø)
pkg/plugin/extension.go	66.66% <100.00%> (+0.95%)	⬆️
pkg/plugin/extension_builder.go	93.54% <100.00%> (+0.44%)	⬆️
pkg/plugin/manifest/convert.go	88.63% <100.00%> (+0.21%)	⬆️