Add start time to change feed query

[bambriz]

Description

Adds Start Time option for change feed queries in Python SDK. A utc datetime is passed into the options when calling a change feed pull. If the datetime is not utc but has a timezone defined it will convert it to utc, otherwise it is assumed to be in UTC. An incorrect value results in an AttributeError.

The following Example creates 10 items at two different times. By using start time you can filter out the items created after the time recorded.

    batchSize = 10
    container = database.get_container_client(container_name)
    create_random_items(container, batchSize)
    sleep(1)
    now = datetime.now(timezone.utc)
    sleep(1)
    create_random_items(container, batchSize)
    change_feed_iter = container.query_items_change_feed(start_time=now)

This PR also fixes a bug where change feed pulls in async clients were failing to fetch remaining pages and only returning the first one by removing the casting to Dict for response headers and keeping them as CIMultiDict type.

This PR was merged into PR 35090 Please refer to that PR.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-cosmos

[annatisch]

Why is this logic different from the async client?
Additionally - it looks like these parameters are mutually exclusive, so what happens if I pass both a start_time as well as start_from_beginning=True? Will the service raise an exception?