[SDK-3768] Add tree-shakable Base* and Rest exports

[lawrence-forooghian]

This PR replaces #1400. It copies a lot from that PR but uses the class hierarchy described by #1415.

See commit messages for more details.

The output of npm run modulereport is as follows:

BaseRest: 74.28 KiB
BaseRest + Rest: 91.14 KiB
BaseRealtime: 148.03 KiB
BaseRealtime + Rest: 156.68 KiB

(Note that BaseRest is currently somewhat larger than BaseClient in #1415. This is because it includes the transports and EventEmitter, which are included due to this line. This line is absent in #1415 because that PR takes the approach of stripping everything out to add it back in again later, whereas I've taken the approach of keeping everything as it is until we explicitly decide to split it out. We'll be splitting out the transports in #1394.)

Resolves #1415.

[owenpearson]

Code changes look fine, I thought we opted not to export a rest client without rest apis though - is there any reason someone would want to use a rest client but not call any rest apis?

[lawrence-forooghian]

Code changes look fine, I thought we opted not to export a rest client without rest apis though - is there any reason someone would want to use a rest client but not call any rest apis?

Yep, you're right. I'll look into what the best way to handle this is — we want a way for BaseRest to include Rest without it being included in its subclass BaseRealtime.