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[bot]]

Codecov Report

:x: Patch coverage is 85.71429% with 6 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 77.57%. Comparing base (dad989c) to head (bfd967f). :warning: Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/datastore/datastore.go	85.72%	5 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2604      +/-   ##
==========================================
+ Coverage   77.07%   77.57%   +0.51%     
==========================================
  Files         466      471       +5     
  Lines       49199    49606     +407     
==========================================
+ Hits        37914    38476     +562     
+ Misses       8502     8279     -223     
- Partials     2783     2851      +68     

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