Automatic merge from submit-queue

Allow specifying secret data using strings

This PR allows specifying non-binary data values in `Secret` objects as `"stringData":{"key":"string value"}`, in addition to the existing base64 []byte serializations in the `data` field.

On write, the keys and values in the `stringData` field are merged to the `data` map, overwriting any values already present in the `data` map. The move is one-way, the `stringData` field is never output when reading from the API.

A Secret could be created like this:
```
{
  "kind":"Secret",
  "apiVersion":"v1",
  "metadata":{"name":"mysecret"},
  "data":{
    "image":"<base64-encoded-jpg>"
  },
  "stringData":{
    "username": "myuser",
    "password": "mypassword"
  }
}
```
and when read from the API would look like this:
```
{
  "kind":"Secret",
  "apiVersion":"v1",
  "metadata":{"name":"mysecret",...},
  "data":{
    "image":"<base64-encoded-jpg>"
    "username": "bXl1c2Vy",
    "password": "bXlwYXNzd29yZA=="
  }
}
```