Automatically start Streamlit process without calling open_panel

[mikeprosserni]

What does this Pull Request accomplish?

This addresses a few points of feedback from Cifra:

• Rename "Open" and "Close" to "Start" and "Stop"
• Return the panel URL from start_panel()
• Remove start() and stop() from the Panel interface. Measurement scripts shouldn't call them.
• • Also remove with support (enter and exit methods)
• Automatically call client.start_panel() from the StreamlitPanel constructor
• Add a readonly panel_url property on StreamlitPanel, so the measurement script can display it

Why should this Pull Request be merged?

Implements AB#3145081

What testing has been done?

All the autotests have been updated

[github-actions]

Test Results

 10 files  ± 0   10 suites  ±0   11s ⏱️ -1s
 65 tests  -  4   65 ✅  -  4  0 💤 ±0  0 ❌ ±0 
620 runs   - 40  620 ✅  - 40  0 💤 ±0  0 ❌ ±0 

Results for commit 239185a. ± Comparison against base commit dc53ce3.

This pull request removes 19 and adds 15 tests. Note that renamed tests count towards both.

tests.unit.test_panel_client ‑ test___open_panels___close_panel_1_with_reset___enumerate_has_panel_2
tests.unit.test_panel_client ‑ test___open_panels___close_panel_1_without_reset___enumerate_has_both_panels
tests.unit.test_panel_client ‑ test___open_panels___enumerate_has_panels
tests.unit.test_python_panel_service_stub ‑ test___open_panel___close_panel___gets_response
tests.unit.test_python_panel_service_stub ‑ test___open_panel___gets_response
tests.unit.test_streamlit_panel ‑ test___different_panels___have_different_panel_ids_and_uris
tests.unit.test_streamlit_panel ‑ test___first_open_panel_fails___open_panel___gets_value
tests.unit.test_streamlit_panel ‑ test___open_panel___panel_is_open_and_in_memory
tests.unit.test_streamlit_panel ‑ test___opened_panel___accessor_set_value___panel_gets_same_value
tests.unit.test_streamlit_panel ‑ test___opened_panel___panel_set_value___accessor_gets_same_value
…

tests.unit.test_panel_client ‑ test___start_panels___enumerate_has_panels
tests.unit.test_panel_client ‑ test___start_panels___stop_panel_1_with_reset___enumerate_has_panel_2
tests.unit.test_panel_client ‑ test___start_panels___stop_panel_1_without_reset___enumerate_has_both_panels
tests.unit.test_python_panel_service_stub ‑ test___start_panel___gets_response
tests.unit.test_python_panel_service_stub ‑ test___start_panel___stop_panel___gets_response
tests.unit.test_streamlit_panel ‑ test___different_panels___have_different_panel_ids_and_panel_script_paths
tests.unit.test_streamlit_panel ‑ test___first_start_will_fail___start_panel___panel_is_functional
tests.unit.test_streamlit_panel ‑ test___panel___accessor_set_value___panel_gets_same_value
tests.unit.test_streamlit_panel ‑ test___panel___get_unset_value___raises_exception
tests.unit.test_streamlit_panel ‑ test___panel___has_panel_id_and_panel_script_path
…