feat: Validate VAR_CONFIG and template variables

[volsa]

Introduces a validation for template variables (e.g. foo AT %I* : BOOL) and their configuration as specified in VAR_CONFIG blocks. Specifically the validation checks

• if the types of the config and template variable match
• if the config variable has specified a full hardware address
• if the template variable has a hardware binding
• if the template variable has specified an incomplete hardware address

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.98990% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.40%. Comparing base (be78df6) to head (bf5f142). Report is 63 commits behind head on master.

Files with missing lines	Patch %	Lines
src/validation/variable.rs	98.87%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1303 +/- ## ========================================== + Coverage 91.11% 91.40% +0.29% ========================================== Files 153 159 +6 Lines 44816 48782 +3966 ========================================== + Hits 40833 44591 +3758 - Misses 3983 4191 +208 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.