Ability to build static sites (SSG mode)

[sgammon]

We should add the ability to build Elide apps into static SSG-based sites. In this mode, requests are executed through @Page-annotated handlers at build time; their local assets are crawled, and the entire resulting set of web assets is considered the output.

This mode would enable Elide to be used as a framework for sites hosted on completely static systems, like Github Pages or Google Cloud Storage/S3.

To do this the right way, we'll need to scan for @Page annotations at build time; two options exist for this, namely, kapt and KSP. Because kapt is deprecated, we should build a KSP processor which produces the appropriate metadata for the SSG compiler.

Rough steps to finish this one:

• Add KSP processor which scans for @Page-annotated handlers and produces an "app manifest," in binary-encoded proto, that describes each of the detected "endpoints."

• Read manifest at build time from the Gradle plugin, when SSG is active, and use it as input to perform the following steps for each listed endpoint, if any (if no endpoints are present, this step is skipped, and an SSG build is not possible):

  • Synthesize a root-page request to the endpoint, save the output
  • Synthesize a request to each mounted asset endpoint, save the output
  • With above outputs, generate a virtual filesystem with index.html / assets

• Write output to filesystem in a structure which mirrors the app

  • For each endpoint, write a folder or index.html or <page>.html
  • For each endpoint, write each asset

• *Compress to tarball which can be uploaded / expanded to hosting systems

  • Best to use a tar.gz probably