Added support for useragent header in outgoing requests

[bhaskarkishore]

Previously all HTTP requests for fetching content did not contain the user-agent header. Omitting this header can potentially cause the destination server to reject the request, thus failing the entire fetch query.

This change adds an optional config item called client_useragent. When a value is set, the header user-agent and the set value will be included in all outgoing HTTP requests.

Closes #43

[codecov-commenter]

Codecov Report

Merging #81 (43107e5) into main (4c4d0d1) will decrease coverage by 0.06%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
- Coverage   24.68%   24.61%   -0.07%     
==========================================
  Files          23       23              
  Lines        1495     1503       +8     
  Branches      319      321       +2     
==========================================
+ Hits          369      370       +1     
- Misses        985      992       +7     
  Partials      141      141              

Impacted Files	Coverage Δ
src/config.rs	0.00% <0.00%> (ø)
src/proxy.rs	15.73% <0.00%> (-0.09%)	⬇️
src/http/hyper_client.rs	23.40% <25.00%> (-0.99%)	⬇️
src/http/mod.rs	47.70% <100.00%> (ø)