Add 'Advanced Config' Button and Menu

[JakeUrban]

Developers will want to tweek the way the demo wallet operates in order to test all edge cases on their anchor.

For example, Settle asked us to add a on_change_callback field to the existing demo client today. The demo wallet should also be able to do that. Note that this includes updating the URL so the config is completely sharable.

[shanzzam]

@JakeUrban Is this still a feature request in the revamped Demo Wallet? If so, where would this be configured @lydiat

[JakeUrban]

I think the plan was to implement this in V2. The idea is that there are optional features provided in the SEPs, and the anchor using our demo wallet will likely want to test the implementation of those optional features by explicitly enabling them.

We could take the approach of defaulting to using this optional features, but then the anchor wouldn't be able to test flows where those optional features are not requested/enabled.

For example, anchors can optionally create claimable balances for users requesting a SEP-24 deposit, but only does so if the wallet uses the claimable_balance_supported: "true" flag in the deposit request. Our current demo wallet passes this parameter, but this means anchors implementing this feature cannot test for the case where the wallet doesn't send the parameter.

If we had an optional config menu, the user could toggle whether or not this claimable_balance_supported flag was used, enabling testing of both flows.