Add format for imf-fixdate used in HTTP headers (#3285)

* Add format for imf-fixdate used in HTTP headers

* Add example

registries/_format/imf-fixdate.md

+---
+owner: mikekistler
+issue: 
+description: date and time as defined by IMF-fixdate - [RFC7231](https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.1.1)
+base_type: string
+layout: default
+---
+
+# <a href="..">{{ page.collection }}</a>
+
+## {{ page.slug }} - {{ page.description }}
+
+Base type: `{{ page.base_type }}`.
+
+The `{{page.slug}}` format represents a date and time as defined by IMF-fixdate - [RFC7231](https://datatracker.ietf.org/doc/html/rfc7231#section-7.1.1.1).
+
+Example: "Sun, 06 Nov 1994 08:49:37 GMT"
+
+This is the preferred format for dates passed in HTTP headers.
+
+{% if page.issue %}
+### GitHub Issue
+
+* [#{{ page.issue }}](https://github.com/OAI/OpenAPI-Specification/issues/{{ page.issue }})
+{% endif %}
+
+{% if page.remarks %}
+### Remarks
+
+{{ page.remarks }}
+{% endif %}