Support token price oracles in a generic way

[chuck-h]

Is your feature request related to a problem? Please describe.
Tokens which are added through the master token list mechanism should display a fiat equivalent balance similar to existing tokens, without requiring token-specific changes to the Light Wallet code. For example, local currency tokens being used at a community-agreed equivalent USD rate should show this in LW.

Describe the solution you'd like
A new field in the TokenModel structure pointing to a price oracle; the field can be populated via the tmastr.seeds metadata. The price oracle interface should support the existing mechanisms for SEEDS, HUSD, and Telos, and also support a community currency with price defined by "centralized control". (Details TBD in coordination with tmastr.seeds and rainbow.seeds contracts.)

Additional context
Add any other context or screenshots about the feature request here.

@JGDoh @RaulUrtecho