Add validation support for FixedBackOff and ExponentialBackOff

[snicoll]

When creating a BackOff instance via RetryPolicy, we get validation for the individual attributes. However, when creating the BackOff implementation manually, there's no such validation.

This makes the use of the implementation a bit cumbersome as we have to replicate the validation rules the builder offers.

[baezzys]

Hi, @snicoll I'd like to work on this issue. This would be my first contribution to Spring Framework.

Can I give it a try?

[sbrannen]

Hi @baezzys,

Thanks for the offer!

However, we have not yet decided exactly how we want to implement this, and if we decide to implement #35356 we may want to do that first before introducing validation.

In light of that, I think it's best if we let the core team implement this one.

Regards,

Sam