Add CMake options to selectively disable different components

Not all SPIRV-Tools components are required by all projects, so having options to selectively enable/disable them would be helpful to optimize build. Something like:

```cmake
option(SPIRV_TOOLS_ENABLE_OPT "Enable SPIRV-Tools-opt" ON)
```

Right now we have to manually exclude generated projects from the build:

```cmake
set(SPIRV_TOOLS_EXCLUDE_TARGETS SPIRV-Tools-link SPIRV-Tools-reduce SPIRV-Tools-shared)
set_target_properties(${SPIRV_TOOLS_EXCLUDE_TARGETS} PROPERTIES EXCLUDE_FROM_ALL 1 EXCLUDE_FROM_DEFAULT_BUILD 1)
```