Event Grid readme and samples issues

[v-xuto]

Section link:

Suggestion:
Add import package

import uuid
import datetime as dt
from msrest.serialization import UTC

Section link:

Reason:
azure.core.exceptions.HttpResponseError: (BadRequest) Invalid key in aeg-sas-token. Valid format :r=UrlEncodedResource&e=UrlEncodedUtcExpiration&s=UrlEncodedSignature. Input sas string: lrPUpeSQUJEMj3paatULJ1ejvtgMvU4PIy3o+sypQ0w=.

Suggestion:
EVENTGRID_SAS is not an access key, but a shared access signature.
But we unable to find SAS (shared access signature) in event grid topic from the 'Azure Portal'. Only find the access key.
Maybe we should consider this method: sas = generate_sas(endpoint, topic_key, expiration_date_utc) to generate sas.

@jongio for notification.

[rakshith91]

Thanks for the issue!

For section2, it is intentional that we did not add a generate_sas

the reason is that it is highly unlikely that someone generates a sas and uses it for themselves. So, we have two different examples for using a sas and generating a sas (it's a cross language feedback that was received in arch board)

point 1 sounds fair to me - thank you!

[rakshith91]

I've updated the description of EVENTGRID_SAS to make it more clear and fixed the first one. Thanks again
https://github.com/Azure/azure-sdk-for-python/pull/17641/files