Downloading of PDFs etc is deferred/lazy #1104

Describe the bug
views.populate_pdf and similar methods do not really populate the PDF but instead only populate a getter method which is later used upon property access. This has multiple drawbacks:

1. The PDF is loaded multiple times if accessed more than once.
2. Access to the PDF is no longer possible after closing the connection to the server
3. Parallelizing the PDF fetching is really hard and unintuitive as the action does not happen where it should be but only later as a side effect of accessing the property. Property access should be as fast as possible and synchronous.
4. Somewhat related to 3., this makes it impossible to create an asyncio wrapper for this library as property access must be synchronous in Python.

Versions
Details of your environment, including:

• Tableau Server version: Tableau Online
• Python version: 3.10.6
• TSC library version: 0.19.0

To Reproduce

with server.auth.sign_in(tableau_auth):
    view = ...
    server.views.populate_pdf(view, pdf_req_option)

pathlib.Path("output.pdf").write_bytes(view.pdf)

Results
It raises an error as it cannot receive the PDF any longer.