Add VARS to each file directory where custom variables can be applied to all files

```
defaults/
└── open_ephys/
├── VARS
├── config.yml
├── config_test.yml
└── setup_abc.json
computers/
└── wdt10burno/
└── open_ephys/
├── VARS
└── config.yml
peripherals/
└── treadmill123.yml
```

VARS is a dict with variables and corresponding values. Example:
```
"treadmill": "peripherals/treadmill123.yml"
"device_a": "com4"
```

Behaviors:
- User pulls ``defaults/open_ephys/config.yml``, for all jinja variables ``{{VARS.treadmill}}`` it will search for the corresponding key in the VARS file in the current directory.
- Need to determine does the variables also follow the same merging pattern as normal config files. Example: if we pull ``/computers/wdt10burno/open_ephys/config.yml`` it will merge with the config.yml in defaults. Will it also merge the two vars (existing keys get overridden, new keys get appended)?
- If we do have this merge behavior for variables, introduce a new endpoint to easily track all variable files that were used for a certain config.