add style customization to hoverlabel

[Lexachoc]

Addresses some requests in #2342 without closing it.

This PR adds several new hoverlabel styling options based on the discussion in #2342.

New options:

• hoverlabel.borderwidth — sets the hover label border width
• hoverlabel.borderradius — sets the hover label border radius
• hoverlabel.borderpad — sets the padding between the text and the border
• hoverlabel.namecolor — sets the secondary hover label text color
• hoverlabel.bgnamecolor — sets the secondary hover label background color
• hoverlabel.shadow — sets a drop shadow on the hover label

Additional improvement

If hoverlabel.namecolor is not specified, the trace color is preserved when it has sufficient contrast against the secondary label background (WCAG 4.5:1). Otherwise, a contrasting text color is used automatically.

(Not sure whether this contrast improvement change belongs in this PR. Happy to move it out if preferred.)

Before/After

See the comparison in #2342 (comment).

[Lexachoc]

Suggested change

	- Add `hoverlabel.borderwidth`, `hoverlabel.borderradius`, `hoverlabel.borderpad`, `hoverlabel.namecolor`, `hoverlabel.bgnamecolor`, and `hoverlabel.shadow` styling options [[#8115](https://github.com/plotly/plotly.js/pull/8115)]
	- Add `hoverlabel.borderwidth`, `hoverlabel.borderradius`, `hoverlabel.borderpad`, `hoverlabel.namecolor`, `hoverlabel.bgnamecolor`, and `hoverlabel.shadow` styling options, and improve text contrast for secondary hover labels [[#8115](https://github.com/plotly/plotly.js/pull/8115)]