Defined CodeGenerator Interface and implement C++

[dbaileychess]

This makes a new CodeGenerator interface that has individual methods for generating code, GRPC, makefiles, etc.. This is to consolidate all the logic of a given code generator in a single location, instead of the current design with free functions.

This is part of a general refactoring of the FlatC.

[enum-class]

is CodeGenrator same as BaseGenerator ?

[dbaileychess]

No,CodeGenerator is a pure interface that combines everything language-specific together. BaseGenerator is an abstract base class that parser-based (not bfbs-based) code generators are written ontop of. At some point, BaseGenerator could implement CodeGenerator interface, but I didn't want to add that now as I would have to touch every language all at once. So I am just moving them over piecemeal.