[koa-pino-logger] Update type definitions (#36682)
[koa-pino-logger] Update type definitions (#36682)

There was an issue with the previous version, which has been remarked
here: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/19448#issuecomment-420932011

The minimal edits I've made fix this issue by extending the
`ExtendableContext` interface instead of `Context`.
georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped