feat(api): add blockchain endpoint

[sleepdefic1t]

Was missing from prior versions.

This PR does the following:

• added Blockchain Path and API Endpoint.
• added Blockchain mock & tests.
• updated CMake files.
• updated documentation.
• updated examples.
• added to changelog.

What kind of change does this PR introduce?

• Feature

Does this PR introduce a breaking change?

• Yes
• No

Does this PR release a new version?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch
• All tests are passing
• New/updated tests are included

[codecov-io]

Codecov Report

Merging #104 into develop will increase coverage by 70.66%.
The diff coverage is 40%.

@@             Coverage Diff             @@
##           develop    #104       +/-   ##
===========================================
+ Coverage    26.93%   97.6%   +70.66%     
===========================================
  Files           14      15        +1     
  Lines          245     250        +5     
===========================================
+ Hits            66     244      +178     
+ Misses         179       6      -173

Impacted Files	Coverage Δ
src/http/os/http.cpp	100% <ø> (+48.88%)	⬆️
src/api/paths.cpp	96.27% <0%> (+96.27%)	⬆️
src/include/cpp-client/http/http.h	100% <100%> (+40%)	⬆️
...c/include/cpp-client/api/blockchain/blockchain.hpp	100% <100%> (ø)
src/include/cpp-client/api/abstract.h	100% <100%> (ø)	⬆️

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