fix(tags): Filter system tags from the tags list

[Vitor-Avila]

SUMMARY

System tags used to be filtered from the Tags list (by filtering tags with : on their name). As a consequence, any user-created tag that included : were not listed as well. #26324 fixed the issue by showing PRs with :, which re-introduced system tags to the table.

This PR adds a custom_tag boolean filter operator to the Tags API so that system tags are properly filtered out from the API response. Filtering these tags in the Client/Frontend side is not effective since your first page might be only system tags, so you would end up with 0 results in the table (and no option to paginate) + you would also end up with inconsistent counts per page.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before

After

TESTING INSTRUCTIONS

1. Make sure that TAGGING_SYSTEM is enabled.
2. Add any account as an owner to any asset.
3. Navigate to Settings > Tags.
4. Validate that the system tag (owner:{{user_id}}) is not listed.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags: TAGGING_SYSTEM
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e86d4d3) 69.48% compared to head (5a0f752) 69.50%.

Files	Patch %	Lines
superset-frontend/src/pages/Tags/index.tsx	0.00%	3 Missing ⚠️
superset/tags/filters.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26701      +/-   ##
==========================================
+ Coverage   69.48%   69.50%   +0.02%     
==========================================
  Files        1894     1895       +1     
  Lines       74138    74155      +17     
  Branches     8243     8243              
==========================================
+ Hits        51515    51542      +27     
+ Misses      20554    20538      -16     
- Partials     2069     2075       +6     

Flag	Coverage Δ
hive	53.85% <66.66%> (+<0.01%)	⬆️
javascript	56.90% <0.00%> (+0.03%)	⬆️
mysql	78.03% <93.33%> (+0.03%)	⬆️
postgres	78.13% <93.33%> (+<0.01%)	⬆️
presto	53.80% <66.66%> (+<0.01%)	⬆️
python	83.01% <93.33%> (+<0.01%)	⬆️
sqlite	77.71% <93.33%> (+<0.01%)	⬆️
unit	56.39% <66.66%> (+<0.01%)	⬆️

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[eschutho]

LGTM. Thanks @Vitor-Avila!