add base to multichain-example

[gagdiez]

Our current multichain example only includes bitcoin and Ethereum, we need to add base as a usable chain

To do:

• Allow to control Base account to make transactions
• Allow to control Base account to call functions on contracts

[PiVortex]

Solana needs a workaround at the moment. I suggest leaving Solana until EDDSA signatures are supported.

Here is a list of chains we can sign transactions on near/NEPs#503
Adding an example of signing transactions on NEAR (two flows: adding a secp256k1 key to an existing named account and deriving an implicit account for a secp256k1) might prove useful also