Add feature to unset environment variable in mirrord.json

[aviramha]

under env we should have unset which would unset given keys.
When executing from VSCode/INtelliJ we join the process env, so the API from ext should probably let the IDE also know that those env should be excluded.

Specific use case was user had AWS_PROFILE, making boto search for a profile locally instead of using service account.
https://discord.com/channels/933706914808889356/1210230207554916383