Vendor easyjson code to fix build

To ensure we can build without easyjson installed, vendor the
easyjson repository as the generated files use the easyjson
library.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

Closes: #1322
Approved by: mheon

vendor/github.com/mailru/easyjson/jlexer/error.go

@@ -0,0 +1,15 @@
+package jlexer
+
+import "fmt"
+
+// LexerError implements the error interface and represents all possible errors that can be
+// generated during parsing the JSON data.
+type LexerError struct {
+	Reason string
+	Offset int
+	Data   string
+}
+
+func (l *LexerError) Error() string {
+	return fmt.Sprintf("parse error: %s near offset %d of '%s'", l.Reason, l.Offset, l.Data)
+}