feat: allow Hangul(korean character) in creating and renaming vfolder

[lizable]

After This PR, the user can create the vfolder contains with Hangul(a.k.a korean character).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (2c17b58) 75.04% compared to head (8dd2be7) 75.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   75.04%   75.07%   +0.03%     
==========================================
  Files          19       19              
  Lines        2500     2504       +4     
==========================================
+ Hits         1876     1880       +4     
  Misses        624      624              

Impacted Files	Coverage Δ
src/ai/backend/common/validators.py	62.60% <100.00%> (+0.42%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[achimnol]

Let's add an option to this Trafaret, ascii_only: bool, so that users can opt-in unicode-aware alphabets.

[achimnol]

Please add the news fragment as well.

[achimnol]

It additionally allows whitespaces compared to the previous version.
Please make it to stand out in the news fragment, and reflect that change in the test cases as well.
Also the code has "ascii_only" argument but the fragment says "unicode" mode -- please make them consistent.

[achimnol]

Let's update as follows:

Update the conditions for Slug for existing use cases:

• disallow the characters outside the regex [\w\-.] (no whitespace allowed!)
• disallow consecutive dots such as .., ..., ... in any position
• disallow zero-length (empty) strings

Add a new validator DirectoryNameComponent for vfolder name validation:

• disallow the characters outside the regex [\w\s\-.] (whitespace allowed!)
• disallow consecutive dots such as .., ..., ... in any position
• disallow zero-length (empty) strings