Simplify Env implementation with python dataclass

[fujikosu]

Summary

This PR adopts dataclass https://docs.python.org/3.7/library/dataclasses.html to Env class. With this change, no need to manually edit 2 places anymore, which are class variable assignment and property creation. One place edit covers both of property creation and variable assignment. This reduces human errors around that.

Before

self._workspace_name = os.environ.get("WORKSPACE_NAME")
@property
def workspace_name(self):
    return self._workspace_name

After

workspace_name: Optional[str] = os.environ.get("WORKSPACE_NAME")

Additional benefit

• Type annotation added to all variables
• frozen=True flag makes this Env's properties immutable

Singlton is deleted

Singlton is deleted for implementation simplification as I didn't see much value of keeping it.

Points considered were these 3 points.

1. Object has resources that are expensive to instantiate
   • No. Every line is just get.environment so it's super cheap to execute
2. Object has resources that should only be instantiated once
   • It's not harmful to instantiate env object multiple times because it's basically just loading environment variables. Cheap & safe read-only
3. Object has resources that should only be de-provisioned when the program ends
   • Ideally we want to keep using the same env throught the codebase but it's fine to create it again if it's lost.

[fujikosu]

@j-so Hi thank you for reviewing this PR! Could you merge this as I don't have permission?