ERC-55 mixed-case checksum address encoding for execution address

[nflaig]

Problem description

Based on discussion in ethereum/beacon-APIs#395 it would be good practice to implement ERC-55 mixed-case checksum address encoding for execution addresses.

There is an open PR to push for a standardization on the Beacon API spec ethereum/beacon-APIs#396, but whether or not this gets merged we might still wanna implement this in Lodestar.

Solution description

We could use a similar approach as Teku (Eth1Address.java), they just extend the Bytes20 SSZ type and override the relevant methods.

Execution address type is defined here

lodestar/packages/types/src/primitive/sszTypes.ts

Line 64 in 6d1dc61

export const ExecutionAddress = Bytes20;

It is only used in three other types

lodestar/packages/types/src/capella/sszTypes.ts

Line 29 in 6d1dc61

export const Withdrawal = new ContainerType(

lodestar/packages/types/src/capella/sszTypes.ts

Line 39 in 6d1dc61

export const BLSToExecutionChange = new ContainerType(

lodestar/packages/types/src/bellatrix/sszTypes.ts

Line 57 in 6d1dc61

export const ExecutionPayload = new ContainerType(

Additional context

There is a related issue regarding execution address formatting #6167. To pass the spec tests we also need the Beacon API spec itself to use consistent formatting.