feat: add eth_addr_checksum validation, update eth_addr

[kamikazechaser]

Rationale

• ethereum addresses are derived from a public key (the last 20 bytes). The network accepts any case insensitive address as long as it is a 40 char alphanumeric (When the last 20 bytes are converted to hex). It is 0x prefixed so that it becomes 42 in length, hence the current eth_addr implementation is modified to reflect this.
• Checksum-ing is optional on ethereum and is a way of typo checking the address hence this is introduced as a new validation i.e. eth_addr_checksum.

refs:

• https://github.com/ethereum/go-ethereum/blob/master/crypto/crypto.go#L275
• https://goethereumbook.org/en/wallet-generate/
• https://goethereumbook.org/en/address-check/
• https://support.metamask.io/hc/en-us/articles/4702972178459-The-Ethereum-address-format-and-why-it-matters-when-using-MetaMask

other lib implemntations:

• https://github.com/chaintool-py/funga-eth/blob/master/funga/eth/encoding.py#L42

Fixes Or Enhances

Fixes #1073

Make sure that you've checked the boxes below before you submit PR:

• [x] Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

Coverage: 74.195% (+0.004%) from 74.191% when pulling e2aaf541d370d8a7994ba9ed2628cc69862b04de on kamikazechaser:sohail/eth-addr into 8f07b0368280d9fc70b4ffd3708b31427bbc286d on go-playground:master.