validate token is actually a token

here is a user who was confused when they tried to add a token that was not the right contract address https://github.com/MetaMask/metamask-extension/issues/10415

theres a couple ways we could attempt to validate
- inspect bytecode for entrypoints (not too hard but doesnt work for proxies and could break with compiler changes or alt langs)
- do a call to a read-only method on the spec. if it fails, dont add it.

Review the user scenario in issue #10415 and compare the two proposed validation approaches: inspecting bytecode entrypoints or calling a read-only method from the token specification. Define which approach should be adopted and what happens when validation fails, including the proxy and alternate-language limitations mentioned here; done means an invalid contract address is not added.

Written by the indexing model from the issue text.