Update Swagger UI and provide dynamic hostname support

[peterbroadhurst]

• Tidies up the OpenAPI generator code in ffapi
• Upgrades Swagger UI to V5
• Exposes on the ffapi.APIServer on /api/openapi.yaml as well as /api/openapi.json
• Allows an option for dynamically determining the public hostname from the request
  • For scenarios where there is complex API Gateway infrastructure in front of the FireFly server, that needs to propagate the external protocol/hostname/path via Header on each request

Note there are breaking API changes for any package that is directly using the SwaggerUIHTML function, which to my knowledge is only FireFly Core (where I'm working on a corresponding change).

There shouldn't be any breaking changes for consumers just using ffapi.APIServer to expose their routes as REST.

[codecov-commenter]

Codecov Report

Merging #97 (151b544) into main (8401dd6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #97   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         4993      5041   +48     
=========================================
+ Hits          4993      5041   +48     

Files Changed	Coverage Δ
pkg/i18n/en_base_config_descriptions.go	100.00% <ø> (ø)
pkg/ffapi/apiserver.go	100.00% <100.00%> (ø)
pkg/ffapi/apiserver_config.go	100.00% <100.00%> (ø)
pkg/ffapi/handler.go	100.00% <100.00%> (ø)
pkg/ffapi/openapi3.go	100.00% <100.00%> (ø)
pkg/ffapi/openapihandler.go	100.00% <100.00%> (ø)