Add external session and context manager to Client

[atimin]

Closes #71

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CHANGELOG.md have been updated (for bug fixes / features / docs)

What kind of change does this PR introduce?

Feature

What is the current behavior?

We create a new session for each HTTP request. This is very inefficient.

What is the new behavior?

To avoid breaking compatibility, I've added an additional argument to constructor to pass an external aihttp.Session. A user can also use a context manager:

    async with Client(url, api_token=api_token) as client:
        bucket = await client.create_bucket("bucket-1", exist_ok=True)
        await bucket.info()

Does this PR introduce a breaking change?

No

Other information: