Support for default dimensions

Hi, sorry in advance if this is already something supported by the library and I couldn't find it.

## What?

I'm trying to override the default dimensions to remove things that aren't useful to me, such as LogGroup and ServiceType.

I can't find a way to actually do this. I see from the Configuration in the README that I can set [namespace](https://github.com/awslabs/aws-embedded-metrics-node/blob/master/README.md?plain=1#L379-L395) which is also useful, but not enough.

I thought there'd be a similar way to handle Dimensions, but there aren't. Ideally, I'd like to be able to create a single MetricsLogger in my application and wire it in as needed. That logger should always have the dimensions I want as baseline, and some methods might add more.

I recognize that this wouldn't work for an environment variable configuration, but how about something like:

```js
const { Configuration } = require("aws-embedded-metrics");

Configuration.dimensions = [{
version: 'N+1' // Latest Lambda Version specific
},
{} // Whole fleet metrics
];
```

This would allow me to alarm on both the "latest" Lambda version specific metrics, and the fleet as a whole as I'd be emitting metrics for both at the same rate. This is especially useful during deployments as I want to see how the newest version is doing.

An alternative would be to open up the [constructor of the MetricsLogger](https://github.com/awslabs/aws-embedded-metrics-node/blob/master/src/logger/MetricsLogger.ts#L33-L37). I see that it is technically public today, but we don't have access to the EnvironmentProvider (again, as far as I am able to tell).

Let me know what you think, and if there's already a way to do this I apologize in advance.