Vendor mongoc and build it with SwiftPM

Currently, the driver links to system-installed libmongoc. We should vendor libmongoc into the driver and build it with SwiftPM instead. Some benefits:

• Better user experience. Currently users need to separately install libmongoc themselves with a package manager, or compile and install it themselves for Linux users. If we built it ourselves users would just need to include the driver.
• We could make our patches to libmongoc for changes that we want/need, but libmongoc will not make for BC reasons etc. 
• We won't have to document a minimum required version of mongoc and will be free to bring in new versions of mongoc at any time.