cp: add initial_balance to content message

[erwanor]

Describe your changes

This adds an initial_balance value to the genesis content message for the CP component.

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

  Consensus breaking

[conorsch]

There's no cli integration, but with the logic added, manual munging of a genesis file would be honored. We've now got a failing test, though:

 thread 'app_can_propose_community_pool_spends' panicked at crates/core/app/tests/app_can_propose_community_pool_spends.rs:305:5:
assertion `left == right` failed: the community pool should be empty at the beginning of the chain
  left: {passet1984fctenw8m2fpl8a9wzguzp7j34d7vravryuhft808nyt9fdggqxmanqm: 0}
 right: {}

which looks rather superficial to fix: the definition of "empty" is should be summing the values of any assets and asserting zero, not assuming no assets are in there.