OnOffCluster: add an optional callback to new()

[lucasvr]

Users of rs-matter may want to execute their own logic when a cluster switches state. This commit adds a callback: Option<&'a dyn Fn(bool)> parameter to OnOffCluster::new(). If set, that callback executes each time OnOffCluster::set(value: bool) is called with a different value than the previous one.

[ivmarkov]

I don't think we should do this. The OnOffCluster handler is just a "sample" handler which is not intended for production use. (So actually I would accept a patch that documents this.)

In general - and putting aside the system / Matter clusters on Endpoint 0, the philosophy of the rs-matter crate is that with regards to cluster callbacks (i.e. things that implement the Handler or AsyncHandler traits), these things have to be implemented by the user itself, and there is not much point in providing "ready made" trait implementations, because these tend to be of little use, and in fact end up as being "double callbacks", just like what you just suggested:

• OnOffCluster already implements the Handler trait, which has three callbacks: read, write and invoke.
• You are suggesting to implement another callback, as part of this callback

What @andreilitvin was recently working on is much more promising IMO:

• In addition to having the generic Handler and AsyncHandler traits, we can have cluster-specific "callback" traits that provide a callback API which is specific for the concrete cluster, and captures in the type system e.g. the exact attribute value being read/written, as well as the e.g. the concrete command being invoked, along with its parameters and return data
• More info here

[lucasvr]

Thanks for the pointer. No worries, let's wait for his PR to land then. I'll close this one for now.