Include receipt evidence in snapshot

[jumaffre]

Part of #2958

Instead of having to verify the evidence of a snapshot in a ledger suffix, which complicated operations, the receipt of the evidence is now directory included in the snasphot file, making them self verifiable.

See inline comments for more details.

TODO:

• Include endorsed node certificate in receipt (see Node certs in receipts #2991)
• Verify receipt in ledger.py and snapshot_serialise.h
• Changelog
• Docs
• On join, verify node certificate endorsement with networkcert.pem (follow-up PR). Also use new flag in read_ledger.py.

[jumaffre]

/azp run

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