Support adding a subtitle inside the h1 in page header block

[andybroomfield]

Ref to

• Consider stacked h1 pattern for guides localgov_guides#122
• 122 consider stacked h1 pattern for guides localgov_guides#133
• Makes page header block lede rendering slightly more flexible localgov_base#492
• https://github.com/localgovdrupal/localgov_publications/blob/1.x/src/Plugin/Block/PublicationPageHeaderBlock.php

There seems to be several ways of trying to get a pattern of having a subtitle that is not the summary to be inside the H1 or look like it is.
It would be better to have an official way inside the page header block, with a setSubtitle() method. This would allow consistent theming to be applied for all subtitles, whilst retaining the ability for modules that wanted to send a render array to replace the title being able to do so.