Add Netlify Functions Context to Netlify Adapter

[jsandler18]

Netlify Functions' Handler type[1] has a context argument that the adapter currently drops. The context contains useful information that Endpoints may want to have access to, namely auth information[2]. This PR passes that context down to Endpoints via app. Doing this has been discussed previously in #1249, including one person patching something roughly equivalent to this change into their local repo.

[1] https://github.com/netlify/functions/blob/efcf21eb6c969c73d165e01b00f33b53f3e150b5/src/function/handler.ts#L11

[2] https://docs.netlify.com/functions/functions-and-identity/

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: b92064e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-netlify	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[benmccann]

I would expect some change to https://github.com/sveltejs/kit/blob/master/packages/adapter-netlify/README.md to document this

[jsandler18]

I would expect some change to https://github.com/sveltejs/kit/blob/master/packages/adapter-netlify/README.md to document this

Done.

[Rich-Harris]

thank you!