feat: add backend dependency type to info.xsd

[st3iny]

For nextcloud/server#46510

[bigcat88]

Maybe <xsl:copy-of select="backend"/> should be added to pre-info.xslt (line 138)?
Or does this work without adding it?

[st3iny]

Talked to Alexander about this and we don't require to change the xslt files for now. They are only required if fields are to be used from the python code.

For now, this field is meant to be consumed by server code only.

[github-actions]

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