add an extension point in bootstrap.toml for metadata?

Summary

Ferrocene maintains a fork of bootstrap with different configuration options than upstream. We put these under a [ferrocene.*] table, which means that using a ferrocene-specific config.toml with an upstream branch causes bootstrap to error.

Does t-bootstrap have any interest in adding a [metadata] field with arbitrary data here so that we can hook onto it, like package.metadata in Cargo.toml? We don't need you to do anything at all with it, just parse the TOML and discard it. I'm happy to make a PR implementing the code.

Command used

x build

Expected behaviour

[custom options are ignored]

Actual behaviour

ERROR: Failed to parse '/Users/jyn/src/rust-upstream/config.toml': unknown field `ferrocene`

Bootstrap configuration (bootstrap.toml)

[ferrocene.example]
option = true

Operating system

macOS Version 15.5 (24F74) Darwin 24.5.0

HEAD

2adaa5dae2b9b63a50fa0200650aeee918cb76fb

Start by locating the bootstrap.toml parsing entry point used by x build and review how unknown tables currently produce the reported error. Implement support for arbitrary metadata that is parsed and discarded, then verify that the shown [ferrocene.example] configuration no longer fails while existing configuration behavior remains unchanged.