Active Target does not allow special or multiple targets

Once the extension has built up its target list it's only possible to select one of the offered targets which are somehow filtered it seems.
So I can neither set e.g. ninja folder targets like src/all nor can I give multiple targets to build exactly what I need for my work.
If the target is not in the list it should just be accepted as-is and passed to ninja.

### Platform and Versions

- **Operating System**: Win10 Remote WSL Ubuntu 20.04
- **CMake Version**: 3.17
- **VSCode Version**: 1.47
- **CMake Tools Extension Version**: 1.4.1
- **Compiler/Toolchain**: clang-10

Start by tracing active-target selection in the CMake Tools extension and how selected targets are passed to ninja; no specific file or test is named. Done means special targets such as src/all and multiple user-supplied targets are accepted and passed through instead of being limited to the offered list.

Written by the indexing model from the issue text.