Make shielded balance more private

[zancas]

Continuing review, I am refining the balance checking fns a bit.

Builds on #1194

• make shield_balance a private inner method
• make some wrappers generic
• remove kruft unused address parameter
• move get_wallet_capability our of "describe"

[fluidvanadium]

agree with Oscarz requested changes, otherwise gtg

[zancas]

I addition to the original changes I changes legacy to concrete as the complement to generic.

[zancas]

Hi @fluidvanadium can you please re-review this?

[zancas]

OK, I have fixed merge conflicts and rerun all tests locally. @fluidvanadium