Enable dynamic header/footer generation in HTML5 output

## Description
Enable generating dynamic (content specific) header/footer generation in HTML5 output.
This is based on the user request in dita-ot-users@googlegroups.com.

Need to write xsl code in custom header.xml using html5 plugin
https://groups.google.com/g/dita-ot-users/c/YocMwRkP6OE

## Possible Solution
Add one extension point to in HTML5 plug-in.

## Potential Alternatives
None.

## Additional Context
Here is sample desired snapshot generating header/footer from input map dynamically.


I will submit pull-request to enable this capability.

Start by locating the HTML5 plug-in and the custom header.xml mentioned in the issue, then trace how the input map is used during output generation. Done means the plug-in exposes an extension point that allows XSL code to generate content-specific headers and footers dynamically, matching the requested sample behavior.

Written by the indexing model from the issue text.