Review `Buffer.from` usage

[nflaig]

Review how Buffer.from is used within Lodestar, and check the following:

• is it required to convert to Buffer at all?
• can we convert to Buffer without memory copy?
• does it impact browser compatibility?

Eg. a good example of incorrect Buffer.from usage https://github.com/ChainSafe/lodestar/blob/252fd2f50a7e759d53983286dcd89f9e7e8d5f60/packages/validator/src/slashingProtection/block/blockBySlotRepository.ts#L57 Which either should not be a Buffer at all as based on type a Uint8Array is valid, or if Buffer is required should avoid the memory copy.

Depending on the size of the Uint8Array, converting to Buffer with memory copy is quite expensive while without copy it's essentially free

From https://github.com/ChainSafe/lodestar/compare/unstable...nflaig/bench-buffer-from

Buffer utils
✔ Buffer.from - copy                  7.286412 ops/s    137.2418 ms/op   x1.024         38 runs   5.82 s
✔ Buffer.from - no copy                1117318 ops/s    895.0000 ns/op   x0.982     723013 runs   1.11 s
✔ Buffer.from - no copy with offset    1179245 ops/s    848.0000 ns/op        -    2699558 runs   3.85 s

Related https://github.com/ChainSafe/lodestar/pull/6687