feat: Custom Metadata and rename arguments

[lambda-science]

This PR is made to improve the converter with latest Haystack convention. Docling has great potential but this implementation seem's to be dead for months.

• It fixes the issue Passing custom metadata per document #8 about passing custom metadata to the converter.
• Rename paths argument to sources to be in line with other converter.
• Changed tests to be real e2e tests without mock to make sure it works fine. To be honest it was just hard to warp my head around the mock to test if my modification were fine.
• Support for Haystack ByteStream sources type

[mergify]

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[lambda-science]

However with this PR I think we loose the ability to put an URL to file as source