[grid] Add part names for table sections (thead/tfoot/tbody)

[stefanuebe]

Describe your motivation

For the sake of completeness it would make sense to not only set parts for rows and cells, but also table sections like thead, to allow easier selection in CSS.

A use case could be to simply hide the header/footer without any server side "remove or reappend the content" setup, but simply having something like

vaadin-grid.hide-header::part(head) {
    display: none
}

or to apply general styling for all header rows

vaadin-grid::part(head) {
    color: salmon; /* hmmmm, salmon */
}

Describe the solution you'd like

thead, tbody and tfoot should have appropriate part names, e.g.

Describe alternatives you've considered

Using Javascript to access the mentioned parts to either apply custom styles or add the part attributes.

Additional context

No response