Fix symbol names containing "eth" to disambiguate between the token and the Ethereum Layer 1

[jpcenteno]

Currently, we have many symbol names containing the word eth referring to the Ethereum L1. ETH is the symbol for the Ether token, while Ethereum is the correct name for the network.

We should use something like l1 and l2 to refer to the blockchain layers, and use eth only when we are talking about the token.

As an example:

• ZKSWallet.eth_balance() returns the ETH balance on the layer-1.
• ZKSWallet.era_balance() returns the ETH balance for the layer-2.
• ZKSWallet.eth_provider is a layer-1 middleware, not something that provides Ether.