Update Linux/MacOS bindings to MySQL client 8.0

[alfonso-eusebio]

This is the PR discussed in diesel's issue diesel-rs/diesel#2373. It includes the update of the bindings to support MySQL client library 8.0 for the common Linux/MacOS bindings_macos.rs file.

Tests:

• This crate - passed on Linux
• Diesel's crate - test passed for the master branch against libmysqlclient20 (MySQL 5.7) and libmysqlclient21 (MySQL 8.0); both on Linux (Ubuntu 18.04 and 20.04, respectively).

Limitations:

• No tests run on MacOS
• This PR does not include an update for the Windows bindings (bindings_windows.rs)

[weiznich]

@sgrif Any chance to get you to have a look a this? I've unfortunately no rights in this repo (and pq-sys as well). It probably makes sense to transfer them both to the diesel organization?

[yerke]

@weiznich Now that you have the rights in this repo, do you want to take a closer look yourself? Thanks for all the hard work.

[weiznich]

@yerke Not for the next for weeks/months as I just don't have the capacity to review and untested +3600 line PR.

[yerke]

@weiznich Perfectly understandable. Thanks for the quick response!

[weiznich]

Closed as duplicate of #40