Implement mempool.get_info Electrum RPC method

[PeterXMR]

Refs #1241 — checks off the mempool.get_info() item on the v1.6 protocol checklist.

Summary

Adds the new Electrum mempool.get_info method introduced in protocol v1.6. Returns the three fee-related fields the spec requires, in BTC/kvB:

Field	Source (bitcoind getmempoolinfo)	Notes
mempoolminfee	mempoolminfee	dynamic — max of minrelaytxfee and mempool minimum
minrelaytxfee	minrelaytxfee	operator-configured
incrementalrelayfee	incrementalrelayfee	omitted from response when bitcoind doesn't expose it (older Cores)

The JSON conversion is factored into a free function mempool_info_to_json so the spec mapping can be unit-tested without an RPC client.

Test plan

• 3 new unit tests in electrum::tests covering the JSON shape: full response, omitted optional field, zero fees
• cargo test --lib — 24/24 pass
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• Live tested against Bitcoin Core v31.0.0 on regtest:

$ printf '{"id":0,"method":"server.version","params":["test","1.4"]}\n{"id":1,"method":"mempool.get_info","params":[]}\n' \
    | nc -w 2 127.0.0.1 60401
{"id":0,"jsonrpc":"2.0","result":["electrs/0.11.1","1.4"]}
{"id":1,"jsonrpc":"2.0","result":{"incrementalrelayfee":1e-6,"mempoolminfee":1e-6,"minrelaytxfee":1e-6}}

Matches bitcoind's reported values:

$ bitcoin-cli getmempoolinfo | jq '{mempoolminfee, minrelaytxfee, incrementalrelayfee}'
{
  "mempoolminfee": 0.00000100,
  "minrelaytxfee": 0.00000100,
  "incrementalrelayfee": 0.00000100
}