Bootstrapping

[bolomcs50]

This PR:

• Introduces bootstrapping, by caching API responses to a file whose name is specified through the new builder method cacheFileName().
• Makes Context a struct, resolving an ambiguity where it was sometimes referred to as a class.
• Renamed the CMake flag ENABLE_TESTING to make it project-specific, making it easier to use the library in another project.

@aruizs I hope this is ok for you, pls let me know if you see something wrong

[aruizs]

Thanks for your contribution @bolomcs50, could you add any test for this feature? Thanks.

[bolomcs50]

@aruizs I modified the existing tests to cover this mechanism too. Basically, now the parametrized tests initializes an unleashClient once to write the mock API response to the cache file and then initializes another one and pretends the API response is empty, thus it's forced to read from the file. This tests both file writing and reading.
Let me know if you think this proper.

P.S. I might have broken the GH actions when renaming the CMake flag, I think this is fixed by adding
-DUNLEASH_ENABLE_TESTING=ON
to the CMake configuration steps for all the OSs.

[bolomcs50]

Hey @aruizs if it's ok with you I'd like open a PR I have ready to add support for constraint operators (essentially covering test set 13 of the specification) once this is merged.

Still reviewing.

[aruizs]

@bolomcs50 sorry for the late review. I'm working on it now. I am checking why the actions are not triggered for your pull requests.

[aruizs]

@bolomcs50 would you mind to rebase and update the PR so that the CI can be launched? Thanks.