feat(`foundryup`): add `version` field to `foundry.toml` ("engines" equivalent) that is selected upon running `foundryup --use`

### Component

Forge

### Describe the feature you would like

[npm has an engines ](https://docs.npmjs.com/cli/v9/configuring-npm/package-json#engines)field that allows you to specify a specific engine such as pnpm, node > 18 ect.

Similarly it would be nice if in a foundry.toml we could specify that foundry has to be pinned to a specific version or higher than a specific version.

When running `foundryup --use` it looks at this version field and then activate the corresponding version, downloading if not available.

This would mirror the behavior of something like `nvm use`.

It is important that a user activates rather than switch automatically.

### Additional notes

One area that may be challenging to implement this is the `foundry-toolchain` as it would need to be aware of the `version` field in `foundry.toml` to select the version to install.

Start with the foundryup --use entry point and the foundry-toolchain area mentioned in the issue, then trace how foundry.toml is read and how versions are downloaded and activated. Done means a version requirement in foundry.toml is honored when explicitly running foundryup --use, including downloading an unavailable matching version without automatic switching.

Written by the indexing model from the issue text.