Add dynamic public prefixes through constitution

[jeffa5]

Previously the maps for kv would be either all private or all public. This makes it more dynamic, moving the option into a constitution-managed map.

Every key is kept in the private map, keys that match the public prefix will also be kept in a public map which is viewable in the ledger. By default there are no public prefixes and so everything is private.

Fixes #179

Code caveats:

• Currently no way to have the CCF sandbox start with a custom constitution https://github.com/microsoft/CCF/issues/4572

[jeffa5]

I think it would be much preferable to have this as constitution-first rather than going with this intermediate step but at least this shows a potential way forward.

[jumaffre]

I think we should add a comment somewhere that this is a bit of a hack, and that the nicer solution to avoid duplicate storage would be to add an iterator API to the CCF kv::Map so that we can foreach across two maps (private and public).

[github-actions[bot]]

Plots from benchmarking at 29b2e0d6fedc70053a09e2aa3db49865b607bb35

Available from here

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[jeffa5]

Needs to wait for a new CCF release so that we have the cose tooling