fix: RND-120: Redis storage doesn't have db field and redis target misses validation_connection()

[makseq]

1. Add Database number (db) on the Redis storage form:
   

2. Add simple validate_connection() for Redis target storages.

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

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[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.66%. Comparing base (9663da0) to head (37d3e0c).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
label_studio/io_storages/redis/models.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6255      +/-   ##
===========================================
- Coverage    76.68%   76.66%   -0.02%     
===========================================
  Files          169      169              
  Lines        13698    13702       +4     
===========================================
+ Hits         10504    10505       +1     
- Misses        3194     3197       +3     

Flag	Coverage Δ
pytests	76.66% <25.00%> (-0.02%)	⬇️

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