fix: reuse http client for externaldata requests

[mannbiher]

Instead of creating a new http.Client for each external data request, create it per provider and reuse it.

Fixes https://github.com/open-policy-agent/frameworks/issues/423

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.41860% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 53.09%. Comparing base (76869f8) to head (a369b9d). Report is 13 commits behind head on master.

Files	Patch %	Lines
constraint/pkg/client/drivers/rego/builtin.go	76.19%	7 Missing and 3 partials :warning:
constraint/pkg/externaldata/request.go	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
- Coverage   54.68%   53.09%   -1.59%     
==========================================
  Files          71      104      +33     
  Lines        5241     6537    +1296     
==========================================
+ Hits         2866     3471     +605     
- Misses       2073     2704     +631     
- Partials      302      362      +60     

Flag	Coverage Δ
unittests	53.09% <74.41%> (-1.59%)	:arrow_down:

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