winston log provider: add ability to modify config for showing/hiding timestamp

If you choose the `winston` log provider, it adds a timestamp by default. Add an option in the config to not show the timestamp, and also the ability to turn on/off timestamp after initialization.

## Proposed

```javascript
// init winston logger (timestamp is true by default for backwards compat)
const aioLogger = require('@adobe/aio-lib-core-logging')('App', {provider:'winston', timestamp: false })
// turn on the timestamp later
aioLogger.config.timestamp = true // use a setter
```

Start at the winston provider initialization and the logger config handling described in the issue. Check the existing tests for logger configuration; done means timestamp is enabled by default, can be disabled through initialization, and can be toggled after initialization through config.timestamp.

Written by the indexing model from the issue text.