[nextjs/template][sitecore-jss-nextjs] On demand ISR

[addy-pathania]

Adds On-demand ISR feature from Nextjs

• An API route is introduced (/api/revalidate). When triggered this will purge the updated page path on-demand
• We have the traditional ISR(revalidate: 5s) which is enabled by default and now on-demand ISR which can be configured using a webhook.
• Users need to pass a secret and configure their environment variable i.e. ISR_REVALIDATE_SECRET in order to prevent unauthorized user access.
• Users need to setup a webhook to automatically trigger the API route. They need to pass the multiSite or personalize Boolean values through their webhook payload if multi-site or personalize is configured.
• This PR follows the Contribution Guide
• Changelog updated

Description / Motivation

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[ambrauer]

I didn't have time to do a full review, but here is some preliminary feedback

[illiakovalenko]

Approved by mistake, need to resolve comments first :)

[ambrauer]

Just some minor things this round, conditionally approving. Nice work!