[Housekeeping] Expose flyteconsole's ENABLE_GA env var to helm chart

[EngHabu]

Describe the issue

As part of addressing #1442, Console's PR flyteorg/flyteconsole#244 is adding an environment variable to control whether Google Analytics should be enabled or not. We need to make changes to the deployment charts to:

• Expose a flag in the values.yaml to control this env var.. Defaults to false
• Enable the flag by default in values-sandbox
• Update getting started docs to explain this and tell people how to turn it off.

What if we do not do this?

N/A

Related component(s)

flyte

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[EngHabu]

Thank you @evalsocket for picking this up!