curl based implementation for http client api

[lalitb]

This brings the CURL implementation for HTTP Client API, along with few minor changes in API.

TODO -

• Bazel build ( bazel build for curl based http client implementation #390 )
• TLS/SSL Support (Enable http(s) ssl verification for curl based http_client implementation #389 )

[codecov]

Codecov Report

Merging #385 (2568485) into master (88d43a3) will decrease coverage by 0.29%.
The diff coverage is 89.52%.

@@            Coverage Diff             @@
##           master     #385      +/-   ##
==========================================
- Coverage   95.00%   94.71%   -0.30%     
==========================================
  Files         171      175       +4     
  Lines        7186     7581     +395     
==========================================
+ Hits         6827     7180     +353     
- Misses        359      401      +42     

Impacted Files	Coverage Δ
...nclude/opentelemetry/ext/http/client/http_client.h	90.90% <ø> (ø)
...lemetry/ext/http/client/curl/http_operation_curl.h	82.43% <82.43%> (ø)
ext/test/http/curl_http_test.cc	93.50% <93.50%> (ø)
...ntelemetry/ext/http/client/curl/http_client_curl.h	95.00% <95.00%> (ø)
sdk/test/metrics/counter_aggregator_test.cc	98.21% <0.00%> (-1.79%)	⬇️
sdk/test/common/circular_buffer_test.cc	98.97% <0.00%> (-1.03%)	⬇️
... and 2 more

[reyang]

LGTM.

[jsuereth]

This looks fine for CURL, but I have a few asks:

• Can you open a ticket about Bazel support and assign to me? I'll look into it.
• Can you update documentation for how to compile w/ CURL support (with a TODO on bazel). I think adding optional deps like these really need to be well documented on how to consume them, and what level of support they have.

[pyohannes]

Thanks for bringing this on the way. I added some suggestions and questions about points that are unclear to me.

[lalitb]

This looks fine for CURL, but I have a few asks:

• Can you open a ticket about Bazel support and assign to me? I'll look into it.
• Can you update documentation for how to compile w/ CURL support (with a TODO on bazel). I think adding optional deps like these really need to be well documented on how to consume them, and what level of support they have.

Appreciate your help here @jsuereth . I have created the ticket #390 for bazel build, and assigned to you. Please feel free to take it up once this PR is merged. I will add documentation for this dependency, though I have mentioned few details in new ticket which will help you get going.

[lalitb]

Need to remove http_server from code coverage . It seems to be affecting the overall percentage.

[lalitb]

@reyang - We discussed this PR during community meeting. It should be fine to merge it. There is tiny drop in code coverage percentage I believe it should be acceptable.