feat: add AccessContext types

[BSFishy]

Adds types for Cloudflare Access integration into Workers

[BSFishy]

Would like to get the team's thoughts on this name. I am unfamiliar with how these types get injected into user's projects (afaiu, Env gets added globally automatically), so maybe a more descriptive name works better than Identity? Perhaps something like CfAccessIdentity?

[danlapid]

Yeah a more specific name would be appropriate but we should also actually give it a type that represents what it can contain

[BSFishy]

A specific type here is difficult because this type is mostly what the customer sends us from their identity provider. A full type would likely look something like

type CloudflareAccessIdentity = {
  email?: string;
  [key: string]: unknown;
}

At which point I feel like typing it gives no real benefit over object

[petebacondarwin]

AccessContext and Identity are quite general types being defined as top level ambient types. This could cause conflicts in the future: I could imagine Identity could be used in a number of scenarios.
Perhaps prefix them with something like CloudflareAccessContext, etc?

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[BSFishy]

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