Add asset search to spotlight invoice request

**Is your feature request related to a problem? Please describe.**

The [invoice page](https://wallet.bitshares.org/#/invoice/request) located in the spotlight has limited assets to select from for invoices.

**Describe the solution you'd like**
I would like to search the blockchain for alternative assets to request payment with.

**Describe alternatives you've considered**
Use a dedicated merchant app, instead of this spotlight component.

Start at the spotlight invoice request page linked in the issue and trace how the current limited asset choices are loaded and selected. Investigate the existing blockchain asset lookup path before deciding how search should fit the request flow. Done means alternative assets can be found and selected for invoice payment requests.

Written by the indexing model from the issue text.