Refactor js_generic to use common DynamicJSEndpointRegistry hierarchy, and expose as an extensible public header

[eddyashton]

Resolves #6682.

We copied a bunch of functionality from js_generic to DynamicJSEndpointRegistry, so that the latter could be extended by apps to run their own JS endpoints. Eventually we wanted near-identical functionality, so these implementations have a lot of duplication. This merges the 2, deleting the standalone js_generic code and making it instead extend the DynamicJSEndpointRegistry hierarchy. By using its own (not-quite-just-a-common-prefix) table names, and pushing some missing functionality up the stack into DynamicJSEndpointRegistry, we get to delete a load of code and get back to functional parity.

The commits in this PR show slightly cleaner steps than the final diff, but the key changes are:

• DynamicJSEndpointRegistry splits out a BaseDynamicJSEndpointRegistry base class, which is everything-but-the-audit functions/members. The new js_generic subclasses Base..., to confirm it doesn't use/want the audit/record functions. Everyone else extends DynamicJSEndpointRegistry and sees no change. (This one is technically unnecessary, but I did it anyway)
• BaseDynamicJSEndpointRegistry now does 2 more things that js_generic was doing, and it wasn't (beautifully, js_generic built and mostly ran succesfully after deleting all of its code and changing its base class, and then a few e2e tests picked up these behavioural discrepancies):
  • Logs execution time of every JS call, controlled by existing (governance-backed) log_execution_metrics option. I think this will be expected for some deployments directly replacing js_generic with app-space JS, but we may want to add some other controls for turning it on/off.
  • Implements get_allowed_verbs so that HTTP OPTIONS requests (and maybe some other requests?) accurately describe the valid verbs for a given path, even when they're JS, rather than just a generic 404.
• js_generic is exposed in a public header, as a sample registry which could be extended with your own js::Extensions. Called GovernanceDrivenJSRegistry, because nithtwd.