ipip-518: pivot to generic schema with URI support

pivot IPIP from modifying peer schema to introducing a new `generic`
schema that supports URIs alongside multiaddrs. existing clients skip
unknown schemas, so this is backward compatible.

- generic schema: arbitrary IDs (did:key, UUID), duck-typed Addrs
  (multiaddrs and/or URIs), opaque protocol names
- filter-addrs extended to match URI schemes in generic schema
- filter-protocols extended to apply to generic schema records
- servers/proxies must pass-through addrs and protocols as-is
- 10 KiB record size limit
- no gatekeeping: protocol names established by rough consensus,
  no central registry required

Author: lidel