kv-builder: Builder.add() mishandles filenames with equals ('=') signs

### Problem

When Builder.add() is called, it is splitting the values on `=` to look for key/value pairs. This doesn't take into account that `=` is a valid character for a filename, and will mishandle files containing that character. An example of such a case can be seen in [vault#12290](https://github.com/hashicorp/vault/issues/12290)

### Expected Result

When Builder.add(string) encounters a raw value such as `@cn=foo,o=example.com.json` then it should read k/v from file `cn=foo,o=example.com.json` and not set `{"@cn": "foo,o=example.com.json"}`