Add environment to implementationDetails

In #768 I added implementationDetails as a mixin for an experience Event. In this issue, I'd like to add a field to implementationDetails to capture the environment where the data was collected. This is different than the channel types because channel records where the event occurred, but I'd like to record the environment where the experienceEvent was collected.

## What are the schemas that are affected by the issue
schemas/context/implementation-details
schemas/context/experienceevent-implementation-details

## What are examples of products that are impacted by the issue
Experience Platform Web SDK (Alloy)

Review schemas/context/implementation-details and schemas/context/experienceevent-implementation-details, starting with the existing implementationDetails definitions and their schema conventions. Define the environment field in both affected schemas so the collected environment is represented distinctly from channel, then verify the schemas remain valid.

Written by the indexing model from the issue text.