Support setting a NAV attribute on metadata scopes

[iramiller]

Summary

Add the ability to set a NAV on a given scope to indicate its value for TVL

Problem Definition

With the release of 1.17 markers now have a new TVL component that allows users to set information about the prices/value of a given token. Expanding on this capability we need to add a similar value to the metadata/scopes so that it is easy to declare a given value of an asset when it is created (USD).

Proposal

Add an attribute to the scope concept (doesn't have to be directly in the scope proto) which allows a user to assign a USD NAV value similar to the one set for markers. Unlike the markers there will not be an exchange function that updates these values until perhaps at a future date the exchange system is expanded to allow trading of scopes directly (instead of requiring them to be tied to a marker).

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[nullpointer0x00]

Are the scope NAVs only supposed to be in USD? Marker module supports multiple NAVs of different denoms. If we want to do that here, I would suggest not having a proto directly on the scope.

[iramiller]

To start with I don't know that we will need the alternate pricing ... but that is certainly coming, if not in the first revision then in a subsequent one. It probably makes sense to keep the api consistent between the two modules though.