Add header_style argument to RichModelSummary callback

[rusmux]

What does this PR do?

Fixes #15683

Add a color option for the table header of the RichModelSummary callback.

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[Borda]

let's add entry to changelog

[rusmux]

@Borda I amended the commit to include entry in the changelog

[Borda]

@rusmux how is it going here? 🐰

[rusmux]

@rusmux how is it going here? 🐰

Oh, I just saw @carmocca self-assigned it. But he removed it, so I'll look into it now.

[carmocca]

Thanks!