Validation of search Pipeline graph

[tanaysoni]

The Pipeline class allows building a custom search pipeline using components like retriever and readers. Under-the-hood, the components(nodes) are represented as a graph. It would be nice to have validation in place when constructing a pipeline object. For instance, raising an error when three nodes are connected to a query classifier that has only two class labels.

Additionally, a dry-run functionality could be added to make it easier to test out new pipelines.

[tholor]

In order for a proper pipeline validation, we need to first define standardized output types of the nodes and accepted input types (as mentioned also in #821)