Allow configuration of `source_id` and `split_id` in `SentenceWindowRetriever`

[sjrl]

Is your feature request related to a problem? Please describe.
This was a request from @ju-gu where for a specific client they would like to use the SentenceWindowRetriever but they need to pass in custom meta keys for source_id and split_id which are hard-coded in Haystack's SentenceWindowRetriever.

Describe the solution you'd like
I'd like to make these fields configurable so users with custom preprocessing set ups can still benefit from this component. I think we should make two new init params to the component source_id_meta_field and split_id_meta_field that have default values source_id and split_id respectively. This way default behavior stays the same, but then users can customize which meta fields contain this information.