rpc: remove 'exported or builtin' restriction for parameters (#20332)

* rpc: remove 'exported or builtin' restriction for parameters

There is no technial reason for this restriction because package reflect
can create values of any type. Requiring parameters and return values to
be exported causes a lot of noise in package exports.

* rpc: fix staticcheck warnings

rpc/types.go

@@ -97,9 +97,8 @@ func (bn *BlockNumber) UnmarshalJSON(data []byte) error {
 		return err
 	}
 	if blckNum > math.MaxInt64 {
-		return fmt.Errorf("Blocknumber too high")
+		return fmt.Errorf("block number larger than int64")
 	}
-
 	*bn = BlockNumber(blckNum)
 	return nil
 }