request: add support for message in shared dependencies

### Is there an existing feature request for this?

- [X] I have searched the existing issues.

### Command

_No response_

### Description

There should be an option to also generate message while updating `shared dependencies`.

In projects where multiple developers are working, there can be instance where they might not know it's a generated dependency and coming from `melos.yaml`. To ensure no-one modifies the generated dependency directly in the package we should also generate a message mentioning the same.

Similar approach is being used in the Flutter repo
https://github.com/flutter/flutter/blob/master/packages/flutter/pubspec.yaml#L33-L49

eg:
```yaml
name: package_a

environment:
sdk: ^3.3.1 # THIS LINE IS AUTOGENERATED - TO UPDATE MODIFY "melos.yaml"

dependencies:
retrofit: 4.0.3 # THIS LINE IS AUTOGENERATED - TO UPDATE MODIFY "melos.yaml"
```

The message can be configured directly in the `melos.yaml` file.

Start by reading the shared-dependency generation behavior and the melos.yaml configuration, then compare the Flutter pubspec example linked in the issue. Done means generated shared dependencies include a configurable message directing contributors to update melos.yaml rather than the generated package files.

Written by the indexing model from the issue text.