plots: introduce support for images (#6431)

* plots: support images initial

* plots: data: unify PlotsData

* plots: data conversion refactor

* plots: renderers: abstract functionality

* plots: uncommenting tests

* plots: self implement grouping

* plots: rename group

* plots: extract rendering tests

* plots: move rendering tests to unit

* plots: add image rendering tests

* plots: dont use dpath to find vega plot

* plots: test find_vega

* plots: test render function

* plots: move rendering out of repo package

* plots: refactoring

* plots: self review refactor

tests/func/utils/test_html.py

@@ -1,43 +0,0 @@
-import pytest
-
-from dvc.utils.html import HTML, PAGE_HTML, MissingPlaceholderError
-
-CUSTOM_PAGE_HTML = """<!DOCTYPE html>
-<html>
-<head>
-    <title>TITLE</title>
-    <script type="text/javascript" src="vega"></script>
-    <script type="text/javascript" src="vega-lite"></script>
-    <script type="text/javascript" src="vega-embed"></script>
-</head>
-<body>
-    {plot_divs}
-</body>
-</html>"""
-
-
-@pytest.mark.parametrize(
-    "template,page_elements,expected_page",
-    [
-        (None, ["content"], PAGE_HTML.format(plot_divs="content")),
-        (
-            CUSTOM_PAGE_HTML,
-            ["content"],
-            CUSTOM_PAGE_HTML.format(plot_divs="content"),
-        ),
-    ],
-)
-def test_html(tmp_dir, template, page_elements, expected_page):
-    page = HTML(template)
-    page.elements = page_elements
-
-    result = page.embed()
-
-    assert result == expected_page
-
-
-def test_no_placeholder(tmp_dir):
-    template = "<head></head><body></body>"
-
-    with pytest.raises(MissingPlaceholderError):
-        HTML(template)