demo and test.py: remove dbsp python dependency

[snkas]

The goal of this PR is to remove the dbsp python dependency across the demos and test.py.

Largest changes:

• Removes python dbsp dependency from demos and test.py
• Main demo README with introduction and overview
• Adds api_helpers python module for repeated functions
• Replaces --actions argument with a single --start argument for demos where it is possible
• Change the use case of test.py for clarity to focus on testing the various API interactions
• Removes demo shell scripts demo.sh, create_demo_projects.sh, and prepare_demo_data.sh as they are not used elsewhere

Notes:

• The api_helpers crate can be removed once PUT is implemented. Especially the create_* functions. The compile_program and start_pipeline could be rewritten to a few lines that are duplicated.
• Once the Python libraries are refactored out, the integration tests should be expanded to cover more API interactions (future PR)

[ryzhyk]

Looks great!

I think this deserves an entry in CHANGELOG (and release notes): "Create demo pipelines with a single shell command".

[ryzhyk]

It looks like with this PR, anyone should be able to easily download the Feldera repo and run the demos, so it would help to have even more high-level "Getting started" instructions telling people who are not familiar with our internal architecture how to actually create all required services (I guess it's docker compose or start-manager.sh?) or pointing them to appropriate docs.

[snkas]

Good point, we can update the docker compose to have a profile and container entry for each demo , and then standardize how to call them.

It could be already this PR or a next one.

[lalithsuresh]

Even better: add a very short section on the top-level README, right after the instructions for how to bring up the pipeline manager. E.g.:

### Run a demo

Once you've brought up a pipeline-manager using the above instructions, you can run one of our many demos. For example:

$: <one command for the most simple demo>

This command will bring up a program named foo and a pipeline named bar. You can view them at <urls>.

Once you've tried this, we suggest exploring the full list of our demos [here](link-to-this-folder).

[lalithsuresh]

Even better: add a very short section on the top-level README, right after the instructions for how to bring up the pipeline manager. E.g.:

### Run a demo

Once you've brought up a pipeline-manager using the above instructions, you can run one of our many demos. For example:

$: <one command for the most simple demo>

This command will bring up a program named foo and a pipeline named bar. You can view them at <urls>.

Once you've tried this, we suggest exploring the full list of our demos [here](link-to-this-folder).

[snkas]

Updated to use PUT requests instead of the api_helpers library (which is now removed).

[lalithsuresh]

LGTM