Addresses #35118 (review).
ConsumeUniValue isn't that useful as a shared fuzz helper, as it just returns the same hard-coded fields. This PR removes it and updates the ipc target to read a UniValue directly from fuzzer input instead. This is similar to what the parse_univalue target already does, and lets ipc test with different JSON values instead of one fixed value.
The helper created a UniValue with hard-coded constants, which
isn't ideal for fuzz tests. Replace it in the ipc fuzz test with
parsing a UniValue directly from the fuzzed data provider.
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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35653.
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ACK 9f3e427228007de5a47db1c08a361f90e67288f9
code review ACK 9f3e427228007de5a47db1c08a361f90e67288f9