Support custom non-pip plugins whose executables are already present on the system by specifying `executable` but no `pip_url`
This would be a way to support https://gitlab.com/meltano/meltano/-/issues/1175, as an alternative to https://gitlab.com/meltano/meltano/-/issues/2205:
```yaml
extractors:
- name: tap-mssql
namespace: tap_mssql
executable: tap-mssql # or /path/to/tap-mssql
```
Of course, this would make a Meltano project less portable than it otherwise would be, since it'll depend on the executable already being present on the system, and it wouldn't be available in a containerized project unless the Dockerfile was modified to install it and make it available on the `PATH`.
issue