miner: refactor helper functions in worker.go (#21044)

This reduces complexity of some lengthy functions in worker.go,
making the code easier to read.

consensus/clique/clique.go

@@ -137,9 +137,8 @@ var (
 	errRecentlySigned = errors.New("recently signed")
 )
 
-// SignerFn is a signer callback function to request a header to be signed by a
-// backing account.
-type SignerFn func(accounts.Account, string, []byte) ([]byte, error)
+// SignerFn hashes and signs the data to be signed by a backing account.
+type SignerFn func(signer accounts.Account, mimeType string, message []byte) ([]byte, error)
 
 // ecrecover extracts the Ethereum account address from a signed header.
 func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, error) {