bug: Existing pipelines missing filter_policy in yaml files will break when loading

[sjrl]

Describe the bug
Recently FilterPolicy was added in a series of PRs to Retrievers of each Document Store.

However, it was added in such a way that is required to specify a value in the yaml file even though the init methods of the Retrievers have a default value for FilterPolicy.

data["init_parameters"]["filter_policy"] = FilterPolicy.from_str(data["init_parameters"]["filter_policy"])

To Reproduce
Try loading a pipeline from a yaml file with a OpenSearchEmbeddingRetriever without specifying filter_policy. This is an issue with all Retrievers all doc stores.

Describe your environment (please complete the following information):

• OS: [e.g. iOS]
• Haystack version: 2.3
• Integration version: opensearch-haystack 0.8.0

[wochinge]

Big thanks for the catch @sjrl !