Issue 94 | Client: deprecate __enter__ in favor of context

[IceKhan13]

Summary

Client: deprecate enter in favor of context

Details and comments

Following code creates a call to len(context)

with serverless:
    ...

which is raising deprecation warning form ray.

For user clearence we will use

serverless.context()
...
# or
with serverless.context():
    ...

to create context inside programs

Closes #94