feat: support external sections in `foundry.toml`

[mds1]

Component

Forge

Describe the feature you would like

There are various requests and use cases for adding support for custom sections to foundry.toml:

• scopelint is a tool for linting forge projects. Instead of having it's own config file, it would be nice to just let users define the settings directly in the forge config file
• In https://github.com/foundry-rs/foundry/issues/4808, there's requests to support parsing the config file so it can be extended to contain settings for forge deploy or Noir circuits

Currently, you can add a custom section to the config file, but it will trigger a warning:

$ forge test
warning: Unknown section [scopelint] found in foundry.toml. This notation for profiles has been deprecated and may result in the profile not being registered in future versions. Please use [profile.scopelint] instead or run `forge config --fix`.

Therefore, I'd suggest requiring that unknown sections are prefixed with something like external., which makes it clear they are for other tools that extend foundry and we'd suppress the warning for those sections.

For example:

# Default foundry configuration
[profile.default]
optimizer = false

[profile.ci]
optimizer = true

# Custom sections for other tools
[external.scopelint]
some_flag = 1

[external.forge_deploy]
another_setting = 123

Additional context

No response

[re1ro]

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