feat: Add an accessor for the mvid to the Metadata Reader

[Swatinem]

This is on top of https://github.com/getsentry/symbolic/pull/659

[getsantry[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀