Update IBeaconConfig#getTypes

[wemeetagain]

Motivation

Currently, we run into an annoying type situation when trying to perform ssz operations on types from a fork known only at runtime.

eg:

const block: allForks.BeaconBlock = ...;

config.getTypes(block.slot).BeaconBlock.hashTreeRoot(block);
//                                                   ^^^^^
// Argument of type 'allForks.BeaconBlock' is not assignable to parameter of type 'phase0.BeaconBlock & altair.BeaconBlock & phase1.BeaconBlock'.

getTypes returns IPhase0SSZTypes | IAltairSSZTypes ....
This means getTypes().BeaconBlock resolves to ContainerType<phase0.BeaconBlock> | ContainerType<altair.BeaconBlock> ...,
which can be further resolved to ContainerType<phase0.BeaconBlock & altair.BeaconBlock ...>.
This intersection type is not what we want, and the way to resolve the issue is with an additional typecast.

Description

The IPhase0SSZTypes | IAltairSSZTypes ... return type has been replaced with IAllForksSSZTypes, which has relevant types replaced with their allForks variant.

This allows for code like the example above to work without additional casting.

[codeclimate]

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