go: vendor the oapi-codegen cmd

See the comment in tools.go, I cannot fully explain what's happening here.
Somehow, Go 1.14 wants to use the vendored version of oapi-codegen but
without this file, oapi-codegen isn't vendored so the generation fails.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

vendor/github.com/deepmap/oapi-codegen/pkg/util/inputmapping.go

@@ -0,0 +1,60 @@
+package util
+
+import (
+	"fmt"
+	"strings"
+)
+
+// The input mapping is experessed on the command line as `key1:value1,key2:value2,...`
+// We parse it here, but need to keep in mind that keys or values may contain
+// commas and colons. We will allow escaping those using double quotes, so
+// when passing in "key1":"value1", we will not look inside the quoted sections.
+func ParseCommandlineMap(src string) (map[string]string, error) {
+	result := make(map[string]string)
+	tuples := splitString(src, ',')
+	for _, t := range tuples {
+		kv := splitString(t, ':')
+		if len(kv) != 2 {
+			return nil, fmt.Errorf("expected key:value, got :%s", t)
+		}
+		key := strings.TrimLeft(kv[0], `"`)
+		key = strings.TrimRight(key, `"`)
+
+		value := strings.TrimLeft(kv[1], `"`)
+		value = strings.TrimRight(value, `"`)
+
+		result[key] = value
+	}
+	return result, nil
+}
+
+// This function splits a string along the specifed separator, but it
+// ignores anything between double quotes for splitting. We do simple
+// inside/outside quote counting. Quotes are not stripped from output.
+func splitString(s string, sep rune) ([]string) {
+	const escapeChar rune = '"'
+
+	var parts []string
+	var part string
+	inQuotes := false
+
+	for _, c := range s {
+		if c == escapeChar{
+			if inQuotes {
+				inQuotes = false
+			} else {
+				inQuotes = true
+			}
+		}
+
+		// If we've gotten the separator rune, consider the previous part
+		// complete, but only if we're outside of quoted sections
+		if c == sep && !inQuotes {
+			parts = append(parts, part)
+			part = ""
+			continue
+		}
+		part = part + string(c)
+	}
+	return append(parts, part)
+}