Enhance OpenAPI interface, for namespace-local APIs, and extra routing options

[peterbroadhurst]

Currently each namespace has it's own API, but you cannot get an OpenAPI/Swagger definition scoped just to that namespace.

This PR proposes an enhance OpenAPI / Swagger UI interface at the Namespace level, which allows an API to be generated that only contains the API at that level.

It also provides extra dynamic header support for the external URL that an individual namespace has been exposed on, such that the namespace local API can be exposed on a mapped URL/Host.

Depends on hyperledger/firefly-common#97

[codecov-commenter]

Codecov Report

Merging #1400 (56af59b) into main (5418e40) will decrease coverage by 0.02%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1400      +/-   ##
==========================================
- Coverage   99.98%   99.96%   -0.02%     
==========================================
  Files         315      316       +1     
  Lines       22078    22147      +69     
==========================================
+ Hits        22074    22139      +65     
- Misses          2        4       +2     
- Partials        2        4       +2     

Files Changed	Coverage Δ
internal/coreconfig/coreconfig.go	100.00% <ø> (ø)
internal/metrics/private_msg.go	100.00% <ø> (ø)
internal/apiserver/ffi2swagger.go	100.00% <100.00%> (ø)
internal/apiserver/routes.go	100.00% <100.00%> (ø)
internal/apiserver/server.go	100.00% <100.00%> (ø)
internal/apiserver/spi_routes.go	100.00% <100.00%> (ø)
internal/contracts/manager.go	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes