Introduce consistent handling of warm_up for components

[mathislucka]

This issue was brought up by @davidsbatista during the review of

We defer time-consuming or resource expensive operations when initializing components so that loading of a pipeline to validate it is a fast and lightweight operation.

Instead of performing expensive operations in a component's __init__-method, we advise to move them to a warm_up-method. The warm_up-method should only be called when the intention is to actually run a component.

We also advise that components should only perform an expensive warm_up once.
For example, our embedders and rankers only load and initialize their model when it wasn't loaded and initialized before.

In the run-method of our Pipeline, we call warm_up on each component, to make sure that we don't run any components that haven't been warmed up.

However, if a component doesn't respect our guideline of checking if it has been warmed up before, this might cause slow operations to run on every invocation of the pipeline's run-method.

We should agree on an approach to make sure that this doesn't happen.

[sjrl]

@julian-risch I believe your issue from here is also related. Perhaps we can merge the two?