feat: add internal functions:serve load-dist-functions knob

This adds an unstable, undocumented configuration knob that instructs
`netlify functions:serve` to load dist bundles, rather than to build
and serve local functions.

This configuration knob is useful for internal testing of SDK-injected
functions.

Author: ndhoule

src/commands/functions/functions-serve.ts

@@ -50,6 +50,7 @@ export const functionsServe = async (options: OptionValues, command: BaseCommand
   })
 
   await startFunctionsServer({
+    loadDistFunctions: process.env.NETLIFY_FUNCTIONS_SERVE_LOAD_DIST_FUNCTIONS === 'true',
     blobsContext,
     config,
     debug: options.debug,