feat: Support configuring Parseable via TOML config file

[niladrix719]

Fixes #1157

Description

Goal: allow configuring Parseable via a TOML file so users don’t juggle dozens of P_* env vars.

Approach: load a config file before Clap runs, hydrate env defaults, and let CLI/env overrides keep their precedence.

Key changes: new config_loader, --config-file flag, added toml dependency, docs/tests for the workflow

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This PR has:

• [x] been tested to ensure log ingestion and log query works.
• [x] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [x] added documentation for new or modified features or behaviors.

Summary by CodeRabbit

• New Features

  • TOML configuration support for environment defaults (defaults to parseable.toml)
  • New CLI option to specify a custom config file
  • Config values map to environment variables with CLI flags taking precedence
  • Storage backend selection via TOML, with automatic injection when configured

• Documentation

  • Added "Configure with TOML" guide and examples to the README

• Tests

  • Added tests covering config-driven CLI behavior and environment handling

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