feat: Add column description

[datnguye]

resolves #24

Description

• Expose column description
• Export DBML with column description (default to empty when not specified)

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR

[codecov[bot]]

Codecov Report

Merging #26 (1ec10f5) into main (e65c768) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines          681       686    +5     
=========================================
+ Hits           681       686    +5     

Impacted Files	Coverage Δ
...rd/adapters/targets/dbml/dbml_test_relationship.py	100.00% <ø> (ø)
dbterd/adapters/algos/base.py	100.00% <100.00%> (ø)
dbterd/adapters/algos/meta.py	100.00% <100.00%> (ø)
...ests/unit/adapters/algos/test_test_relationship.py	100.00% <100.00%> (ø)