feat: add granular datastore connection flags

[ivanauth]

Fixes #2218

Summary

Adds individual connection parameter flags as an alternative to --datastore-conn-uri for easier configuration from Kubernetes secrets or environment variables.

New Flags

• --datastore-host: database host
• --datastore-port: database port (defaults: 5432 for Postgres/CRDB, 3306 for MySQL)
• --datastore-username: database username
• --datastore-password: database password
• --datastore-database: database name

Features

• Auto-builds connection URI from granular params when --datastore-conn-uri is not provided
• Supports Postgres, CockroachDB, and MySQL
• Full backward compatibility with existing --datastore-conn-uri flag
• URI takes precedence if both provided

Example Usage

spicedb serve \
  --datastore-engine=postgres \
  --datastore-host=localhost \
  --datastore-port=5432 \
  --datastore-username=spicedb \
  --datastore-password=secret \
  --datastore-database=spicedb

[codecov]

Codecov Report

❌ Patch coverage is 89.36170% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.61%. Comparing base (d01abad) to head (2f8ef15).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/datastore/datastore.go	89.37%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2604      +/-   ##
==========================================
+ Coverage   77.59%   77.61%   +0.02%     
==========================================
  Files         442      442              
  Lines       54871    54919      +48     
==========================================
+ Hits        42573    42621      +48     
- Misses       9628     9630       +2     
+ Partials     2670     2668       -2     

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