rpc: expose signet challenge in chain info

[ziggie1984]

Summary

• retain the signet challenge in chaincfg custom signet params
• expose signet_challenge from getblockchaininfo and getmininginfo
• add a temporary local chaincfg/v2 replace for root-module builds until a tagged/pseudo module version is used

Tests

• go test ./... in chaincfg
• go test ./... in btcjson
• go test . in the root btcd module