feat: support custom zig versions and master builds

[liyu1981]

• Add load_custom_versions() to read from ~/.asdf/custom/zig/versions.json
• Include master_ in list-all output for zig master builds
• Handle master_ prefix in download to map to 'master' index entry
• Support downloading custom dev versions from ziglang.org/builds/
• Update README with custom version usage instructions

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Summary by cubic

Adds support for installing custom Zig dev versions and master builds, so users can pin non-official builds like 0.12.0-dev.2139+e025ad7b4.

• Reads custom versions from ~/.asdf/custom/zig/versions.json and shows them in asdf list all zig.
• Lists master_<version> for the current master build and maps it back to the master index entry when downloading.
• Skips shasum verification for custom versions since no checksum is available.
• Documents custom version setup and master installation in the README.

^{Written for commit f2fdf28. Summary will update on new commits.}