Add altair and bellatrix states to era

[acolytec3]

Adds SSZ types for Phase0, Altair, Bellatrix, and Capella BeaconState types to era package to support the various Beacon State objects across forks.

Also, updates the README.md with basic usage for the era file format

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.87%. Comparing base (c3301a4) to head (c42d185).

Additional details and impacted files

Flag	Coverage Δ
block	76.87% <ø> (ø)
blockchain	85.69% <ø> (ø)
client	66.27% <ø> (ø)
common	90.73% <ø> (ø)
devp2p	76.27% <ø> (ø)
ethash	81.04% <ø> (ø)
evm	71.04% <ø> (ø)
genesis	99.84% <ø> (ø)
mpt	59.69% <ø> (-0.04%)	⬇️
rlp	69.70% <ø> (ø)
statemanager	70.47% <ø> (ø)
tx	80.96% <ø> (ø)
util	85.54% <ø> (ø)
vm	57.81% <ø> (ø)
wallet	83.78% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[ScottyPoi]

Tested with era files from different hardforks 👍
Approved 👍