Added new feature of Radial gradient

[GoThrones]

Overview: What does this pull request change?

This PR adds a new feature of having Radial Gradient to any VMobject.

Motivation and Explanation: Why and how do your changes improve the library?

There was no such feature in Manim.

It may also be pertinent to see this PR that I had made about a month ago: #4664
The method therein adds the feature of Radial gradient in a different way.

The code changes of PR #4664 and current PR can both co-exist and serve different purpose.

This is the test code:

class testradialgradient1(Scene):
    def construct(self):
        circle = Circle(radius = 1).set_fill([PURE_YELLOW, GREEN], opacity=1,gradient_type="radial").set_stroke(opacity = 0)
        rect = Rectangle().set_fill([ORANGE, PURE_YELLOW], opacity = 1, gradient_type= "radial").set_stroke(opacity = 0)        
        rp = RegularPolygon(3).scale(1.3*circle.radius).set_fill([BLUE, RED], opacity = 1, gradient_type= "radial").set_stroke(opacity = 0)
        vg = VGroup(circle, rect, rp).arrange()
        self.add(vg)

Output is:

---

class testradialgradient2(Scene):
    def construct(self):
        dots = VGroup()
        for r in np.arange(0.25, 4, 0.5):
            for angle in np.linspace(0, TAU, 12, endpoint = False):
                dots.add(Dot(radius = max(0.05*r, 0.06)).move_to([r * np.cos(angle), r * np.sin(angle), 0]))
        dots.add(Dot(radius=0.05))
        dots.set_fill([RED, PURE_YELLOW], gradient_type= "radial", start_radius = 1)
        self.add(dots)
        self.play(dots.animate.move_to(3*RIGHT))
        self.wait()

Output of this code:

testradialgradient2.mp4

---

class testradialgradient3(Scene):
    def construct(self):
        circle = Circle(radius = 1)
        circle.move_to(3*LEFT+1.5*DOWN)
        circle.set_fill([PURE_YELLOW, GREEN], opacity=1,gradient_type="radial").set_stroke(opacity = 0)
        rect = Rectangle()
        rect.move_to(1.5*UP+3*RIGHT).set_fill([ORANGE, PURE_YELLOW], opacity = 1, gradient_type= "radial").set_stroke(opacity = 0)
        self.play(*[GrowFromCenter(mob) for mob in (circle, rect)])
        self.wait()        
        rect.add_updater(lambda x, dt: x.shift(LEFT*dt))
        circle.add_updater(lambda x,dt: x.shift(RIGHT*dt))
        self.wait(6)

Output of this code:

testradialgradient3.mp4

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## Reviewer Checklist
- [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
- [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
- [ ] If applicable: newly added functions and classes are tested