Add custom header and footer support

[Andrew565]

Description

This ticket is for implementing the discovery work completed in #1828. The developer will add support for custom headers and footers to the vets-website and content-build repos. This support will utilize new registry values - customHeader and customFooter - which will accept a string value representing the name of the header or footer which should be included and rendered. These headers and footers can be added at a later date.

Details

Draft PR for vets-website: department-of-veterans-affairs/vets-website#25275
Draft PR for content-build: department-of-veterans-affairs/content-build#1671

Tasks

• Add customHeader and customFooter support to vets-website's dev-template.ejs
• Create "placeholder" custom header and footer on vets-website in the dev-template.ejs file
• Add customHeader and customFooter support to content-build's header.html and footer.html
• Create "placeholder" custom header and footer on content-build in order to verify that support works there

Acceptance Criteria

• Custom header and footer support works