Introduce TOML-based configuration to replace data.py

[glehmann]

The previous data.py approach had several shortcomings:

• No base configuration: users had to start from scratch or copy data.py-dist
• No environment management: working with multiple infra required to manually
  copy the data.py files
• No clear default and customization separation: the data.py, once copied
  from data.py-dist and customized, was mixing the defaults and the
  customizations

The new TOML config system addresses this by:

• Providing a base config.toml maintained by the project with sensible defaults,
  which can be updated without user action
• Supporting per-environment override files (config..toml) so users only
  specify what differs from the defaults, making their customizations explicit

Options have been grouped by sections for clarity.

Configuration sections and their contents:

• [host]: default XAPI credentials (user, password)
• [hosts]: per-host credential overrides
• [network]: management network name
• [pxe]: PXE server settings (arp_server)
• [vm]: VM settings (default_sr, images, equivalents, def_url)
• [install]: installer settings (answerfiles, iso_remaster, isos)
• [guest_tools]: guest tools packages and installer ISOs (win, other, installed, download_url)
• [ssh]: SSH client settings (pubkey, output_max_lines, ignore_banner)
• [storage]: storage backend configs (nfs, nfs4, nfs_iso, cifs_iso, cephfs, moosefs, lvmoiscsi)
• Root keys: objects_name_prefix, dns_server

Also includes config-schema.json, lib/config_loader.py (Pydantic models),
and scripts/migrate_data_py.py to help users migrate from data.py.

This PR is part of a complex set of PRs containing 6 PRs:

1. master
2. Use passlib in place of legacycrypt #396 → master
3. "Introduce TOML-based configuration to replace data.py" (this PR) → Use passlib in place of legacycrypt #396
4. refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing #492 → master
5. misc type related enhancements #460 → refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing #492
6. chore: add ty type checker and reorder make commands for faster feedback #476 → misc type related enhancements #460
7. Add Docker-based compatibility test kit for external validation #431 → refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing #492, Use passlib in place of legacycrypt #396

[olivierh-pro]

Maybe you should choose a convention "snake_case" or "kebab-case", what do you think?