Templated http_router for broader reuse and using boost-ext/ut instead of openalgz/ut

[arun5309]

I have made http_router more generic by templating the handler type to address issue #1873.

But I had to use boost-ext/ut instead of openalgz/ut as the latter lacked support for parameterized tests to make testing it easier.

I will clean up the commits and possibly move back to openalgz/ut depending on your feedback.

[stephenberry]

@arun5309, this look excellent! Yeah, I should add parameterized tests to openalgz/ut. The reason we can't use boost-ext/ut is that there are critical multi-threading bugs that cause programs to crash by simply including boost-ext/ut: https://github.com/boost-ext/ut/issues/618

[arun5309]

One thing I forgot to mention is that std::move_only_function did not work as a handler (type) that is why I created alt_handler type. Looking into if the handler type needs to be copyable might be of interest.

[stephenberry]

Based on this pull request I implemented this in #2151. So, we can close this now. Thanks for contributing this approach and idea!