Add server rendering service to enable standalone route rendering

[eliperelman]

Summary

Introduces a new backend application service which exposes rendering ability from a registered provider. The new server rendering service exposes a RenderingProvider to core for contextual rendering access during an HTTP request: await context.core.rendering.render(). This can be used directly as the content (i.e. response body) for a bootstrapped page.

Blocks #51527.

TODO

• File an issue to migrate the bootstrap.js endpoint
• File an issue to remove injected vars from KP
• Ping teams to removed injected vars

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

Dev Docs

Render a bootstrapped HTML page from a route handler using the RenderingService from your server plugin:

const router = createRouter();

router.get(
  { path: '/', validate: false },
  (context, request, response) =>
    response.ok({
      headers: {
        'content-security-policy': context.core.http.csp.header,
      },
      body: await context.core.rendering.render(),
    })
);

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[elasticmachine]

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[joshdover]

A few issues I noticed:

• The Kibana logo is missing on the bootstrap loading page spinner
• The frontend fails to boot with this error: A legacy application is already registered with the id "siem" (I'm guessing you already noticed this one though)

[kibanamachine]

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[flash1293]

@eliperelman It seems like there is a small typo in your dev docs example -

        'content-security-policy': context.core.http.csp.header,

should actually be

        'content-security-policy': core.http.csp.header,

At least that's what the types say. Could you confirm?