Revisit Account::initialize_from_components and add AccountBuilder::build_existing

[PhilippGackstatter]

Changes

• Remove AccountBuilder::build_testing and add AccountBuilder::build_existing. This means we have a clear separation now between AccountBuilder::build which builds a new account and is available generally and AccountBuilder::build_existing which builds an existing account and is available only under testing.
• Make Account::initialize_from_components private and replace usages with AccountBuilder. It turns out that we don't need to build accounts with a specific AccountId anywhere in tests so those occurrences were changed to generate a new one instead.
• The nonce field in the AccountBuilder was no longer needed so I removed it. The nonce is set in build and build_existing directly.
• The methods on MockChain to add new accounts were all adding the build accounts to MockChain::available_accounts and some methods also to MockChain::pending_objects which seemed unnecessary. I'm not 100% sure whether this is fine though.

closes #949

Builds on top of #871 and should be merged after that one but is otherwise ready for review.

[bobbinth]

Looks good! Thank you! I left a could of small comments inline - but also, would be great for @igamigo to review the PR.

[bobbinth]

Why do we no longer need this?

[PhilippGackstatter]

Perhaps @igamigo can clarify this. It's not clear to me when adding new accounts in the chain whether they must be added to both available_accounts and pending_objects or only the former.

It seems the former is sufficient, so since we're adding the account to available_accounts adding it to pending_objects seems unnecessary. And we're already not doing it for the "existing faucet" case (this is from current next), so I made it consistent:

miden-base/miden-tx/src/testing/mock_chain/mod.rs

Lines 475 to 479 in 6e0e29f

	self.available_accounts
	.insert(account.id(), MockAccount::new(account.clone(), seed, authenticator));
	MockFungibleFaucet(account)
	}

[igamigo]

Originally the intent was to keep MockChain consistent with how the actual chain would evolve, (currently not entirely the case since some updates are not performed entirely as they should) considering things like updates to the account and note trees. For new accounts, I don't think it's needed to add the account to pending objects, but I believe I originally added it for convenience (since it did not hurt either).

[igamigo]

LGTM!

[igamigo]

Originally the intent was to keep MockChain consistent with how the actual chain would evolve, (currently not entirely the case since some updates are not performed entirely as they should) considering things like updates to the account and note trees. For new accounts, I don't think it's needed to add the account to pending objects, but I believe I originally added it for convenience (since it did not hurt either).