Feature-gate proxy and service API

[zeenix]

Turns out that LTO isn't perfect so even when binaries using zbus are built with lto = fat, they still end up with API they don't use at all. One main example is busd, where the binary ends up not only with proxy code that it doesn't use at all, this code contributes quite significantly to the size of busd binary. So let's feature gate both proxy and service API so pure services can disable all proxy code and vice versa.

Unfortunately, these features will have to be default and adding new default features would be a breaking change. So it'll have to wait till 6.0.

This will be part of the #304 efforts.