Add extra HTTP client / retry options

[peterbroadhurst]

Have a use case where I need to:

1. Generate a new piece of data on each retry (a header has to be updated each time)
2. Do some special checking on status codes etc. to determine if retry is suitable
3. Limit the number of concurrent HTTP socket connections to a host

[codecov-commenter]

Codecov Report

Merging #88 (223178f) into main (da6668c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #88   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         4778      4786    +8     
=========================================
+ Hits          4778      4786    +8     

Impacted Files	Coverage Δ
pkg/i18n/en_base_config_descriptions.go	100.00% <ø> (ø)
pkg/ffresty/config.go	100.00% <100.00%> (ø)
pkg/ffresty/ffresty.go	100.00% <100.00%> (ø)