allow creating custom auth providers

[lygaret]

in order to provide different anthropic credentials depending on the project (eg, when you don't want to use your work's Claude subscription for personal projects), I need a way to utilize this plugin's already existing handling of oauth, and just override the name.

After merging, default behavior stays the same, but custom plugins will be able to create custom anthropic sessions:

---

Example

1. create custom plugin, which uses this package;

// ~/.config/opencode/plugins/anthropic-personal.mjs

import { makeAnthropicProviderPlugin } from 'opencode-anthropic-auth'
export async function AnthropicPersonalAuthPlugin({ client }) {
  return makeAnthropicProviderPlugin('anthropic-personal', client);
}

2. update ~/.config/opencode/opencode.json to reference the plugin and add models

{
  "plugin": [
    "file:///Users/jonathan/.config/opencode/plugin/anthropic-personal.mjs"
  ],
  "provider": {
    "anthropic-personal": {
      "npm": "@ai-sdk/anthropic",
      "name": "Anthropic (Personal)",
      "models": {
        "claude-sonnet-4-5": {
          "name": "Claude Sonnet 4.5"
        }
      }
    }
  }
}

(note that until this PR is merged, the import won't work, so this whole thing will need to go in a full package, with a package.json referencing the fork, etc., once merged, the import will resolve to the bun packaged version, and this should be it.

3. choose a model and connect