refactor(core-state): wallet manager indexes

[spkjp]

The WalletManager has several wallet indexes (byPublicKey, etc.), but it's difficult
to add custom indexes. This PR introduces some new functions to make it easier to create a custom wallet index and also makes the whole indexing more generic in general.

Example:

walletManager.registerIndex("customIndex", (index: State.IWalletIndex, wallet: State.IWallet): void => {
    if (wallet.hasAttribute("custom.attribute")) {
        index.set(wallet.getAttribute("custom.attribute"), wallet);
    }
});

const wallet = new Wallet(...);
wallet.setAttribute("custom.attribute", "something");
walletManager.reindex(wallet);

expect(walletManager.findByIndex("customIndex", "something")).toBe(wallet);

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactor
• Performance
• Tests
• Build
• Documentation
• Code style update
• Continuous Integration
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Does this PR release a new version?

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch
• All tests are passing
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)