`PluginGroupBuilder` needs methods that do not take `self`

## What problem does this solve or what need does it fill?

If you have a plugin that is behind a feature, you can't add it to a plugin group because gating an attribution behind a cfg is experimental
```rust
#[cfg(feature = "a")]
builder = builder.add(MyPlugin); // This is experimental
```

## What solution would you like?

Have variants of the methods that take `&mut self` instead

## What alternative(s) have you considered?

* Adding the gated plugin outside of the `PluginGroupBuilder`
* Some other ugly workaround

Start by locating `PluginGroupBuilder` and reading its existing builder methods and tests. Add equivalent `&mut self` variants that allow feature-gated plugins, then verify the relevant plugin-group tests cover the gated usage and pass.

Written by the indexing model from the issue text.