enable keep alive connections

[eguzki]

For each 3scale Admin API call, the client was opening a new connection. For backwards compat, the default behavior will still be to open a new connection for each API call. But keep_alive new attr allows to keep connections open for multiple calls.

[codecov-commenter]

Codecov Report

Merging #104 (5475b70) into main (2cdce79) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   99.65%   99.66%   +0.01%     
==========================================
  Files          12       12              
  Lines        1439     1495      +56     
==========================================
+ Hits         1434     1490      +56     
  Misses          5        5              

Impacted Files	Coverage Δ
lib/3scale/api.rb	100.00% <100.00%> (ø)
lib/3scale/api/http_client.rb	97.56% <100.00%> (+0.19%)	⬆️
spec/api/http_client_spec.rb	100.00% <100.00%> (ø)

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