feat: Add filename field to Metadata

[lcian]

For Debug Files, the current Sentry endpoint returns a Content-Disposition: attachment; filename="<filename>" header, which prompts browsers to save the file under the given name.
Objectstore doesn't currently return this, so if we want downloads from e.g. the UI to go directly to Objectstore, the user will likely be prompted with the Objectstore key as the filename, which doesn't necessarily match the logical filename we show e.g. in the UI itself.

This introduces a first-class Metadata field to the server and clients, which is send by clients as x-sn-filename and rendered in GET/HEAD responses via such a Content-Disposition header.

This field can be an arbitrary String.
When deserializing it into a Content-Disposition, we perform escaping and normalization of path separators and filenames containing only dots.

Close FS-134

[linear-code]

FS-134

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