Abstract download file extraction and conversion

[navarone-feekery]

Related to https://github.com/elastic/enterprise-search-team/issues/5857

This PR moves the logic for downloading files to temp files, and their extraction/conversion to b64 to BaseDataSource.
The new tool is very generic, so it can be easily added to other connectors in future PRs.
If this is approved, I will also generalise/abstract the file size and file extension check.

This PR uses two connectors as examples for how this tool would be used:

1. Confluence

This is a "standard" download and extraction case. The download func uses response.content.iter_chunked, which is the same pattern that most connectors use. This download func is now abstracted onto the BaseDataSource class as generic_chunked_download_func.

1. Azure Blob Storage

ABS has an irregular download method (it uses a library and has its own chunking method). As a result, its download func can't be abstracted. Instead the download func is defined in the ABS connector and is passed as a partial arg.

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[artem-shelkovnikov]

Good stuff! Will you follow-up with using it in other sources, or we create an issue and address later?

[navarone-feekery]

@artem-shelkovnikov Thanks! I'm intending to use this in all sources with file downloads, I'll be making some batch PRs for that now :)

[github-actions]

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