FindSASL2 cleanup (#1405)

* Redefine the handling of SASL imports

Tweak importing of libsasl2 for user requirements

Tweaks the import of libsasl2 so that at import-time we don't
*require* the pkg-config files, since the dynamic library's dep
on libsasl should be sufficient to satisfy the runtime linking.

* Add FeatureSummary
* Bump Earthly version, allow setting sasl options
* Include dependencies for finding libsasl2
* Define more Earthly tasks
* make _mongo_pick public, tweak signature of bool01
* Add Earthly target for testing against cxx-driver

Build +test-cxx-driver to run a default mongo-cxx-driver using the
local mongo-c-driver as the driver backend.

* Better Earthly target docs
* Fix SASL path finding for Windows, and use parallel Make