Add spec for new History network

[morph-dev]

Note
This is built on top of #409 which should be merged first. Only new commits should be reviewed for this PR.
These two PRs are the results of suggestion on #407

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Defines new "History" network per https://ethresear.ch/t/using-portal-network-protocol-for-history-expiry/22658

Most of the spec is copied from https://ethresear.ch/t/using-portal-network-protocol-for-history-expiry/22658. Few points to highlight:

• Using 0x00 and 0x01 as content key selectors
• Added content_type as an argument in content_id function
  • It's used as the least significant byte of the resulting content id
  • Its purpose is to make unique content ids across different content types (i.e. block bodies and receipts)
• Removed the comparison to legacy history network
  • I can add it back, but I feel it shouldn't be in the spec
• I was thinking of extracting new content_id spec into new file, but decided against it for now