Adding a generic extension which can return arbitrary types

[dgruber]

One limitation of the ExtensionList is that it accepts/returns only string based values.

// Extension is a struct which is embedded in DRMAA2 objects
// which are extensible. The extension is named in the DRMAA2
// spec as SetIntanceValue / GetInstanceValue.
type Extension struct {
ExtensionList map[string]string // stores the extension requests as string
}

While strings can be converted to different types it would be required in some situations
to return different types, like implementations of generic interfaces.

To prevent making breaking changes a GenericExtension could be added which
defines a map[interface{}]interface{}.