Create a "Scan for Tokens" button

**What problem are you trying to solve?**
Currently we have a repository that may not have tokens our users are looking for. They are then required to manually enter the token via the Add Token -->Custom Token section (which requires them to gather information from external sites). If done right this could lower our API calls to infura.

**Describe the solution you'd like**
Offer a "Scan for Tokens" button, which scans common sites and repositories that users are already going to anyways for the contract data, decimals, and symbol. (eg. etherscan, [mycrypto repository](https://github.com/MyCryptoHQ/MyCrypto/blob/develop/common/config/tokens/eth.json), coingecko, etc.)

**Additional context**
Maybe offer a disclaimer that when someone uses this button, they are relying on 3rd parties (list those parties, and to only use it if they trust those sources). Dan F. suggested we maybe offer a list of sources for them to pick from.

Example flow: mycrypto's scan for token button

The issue names no repository files, tests, or entry points. Start by reviewing the proposed sources—Etherscan, the MyCrypto token repository, and CoinGecko—and clarify the scan flow, source selection, third-party disclaimer, and completion criteria before locating the relevant token-management UI.

Written by the indexing model from the issue text.