Allow multiple dropdown in Web UI

[jFompe]

Addresses #3260

This adds web UI support for a dropdown list in custom arguments with multiple selection:

Code:

parser.add_argument("--endpoints", choices=["status", "test", "users"], is_multiple=True, default=["status"], help="Endpoints")

Testing

I ran locally with run-local-web.sh and run-local-headless.sh on Ubuntu 24.04 in a virtual env with Python 3.12.3.

Also I added 3 tests to test_web.py and ran the file with pytest locust/test/test_web.py. All the tests passed.

Also used this code to check that the parsed value was always the expected one:

@events.test_start.add_listener
def on_test_start(environment, **kwargs):
    endpoints = environment.parsed_options.endpoints
    print(f"Argument endpoints. Type: {type(endpoints)}. Value: {endpoints}")

Tests with run-local-web.sh

Web UI mode, select multiple:
Argument endpoints. Type: <class 'list'>. Value: ['status', 'test', 'users']

Web UI mode, select one:
Argument endpoints. Type: <class 'list'>. Value: ['users']

Tests with run-local-headless.sh

Headless mode, select one.

"${VIRTUAL_ENV}/bin/python" -m locust \
  --locustfile "${script_dir}/locustfile.py" \
  --endpoints users \
  --headless \
  --host "https://example.com" \
  --loglevel "INFO"

Argument endpoints. Type: <class 'list'>. Value: ['users']

Headless mode, select multiple.

"${VIRTUAL_ENV}/bin/python" -m locust \
  --locustfile "${script_dir}/locustfile.py" \
  --endpoints users status \
  --headless \
  --host "https://example.com" \
  --loglevel "INFO"

Argument endpoints. Type: <class 'list'>. Value: ['users', 'status']

NOTE: for using this in headless mode it is necessary to add nargs='+' in add_argument.
For example:

parser.add_argument("--endpoints", nargs='+', choices=["status", "test", "users"], is_multiple=True, default=["status"], help="Endpoints")