Fix authentication issues with the http-requests against the proxy

[ivangonzalezacuna]

The handler was using the old authentication setup instead of the new one using the new auth service. Let's use this instead and always set the token if defined. This is following the same idea as other upstream actions, such as the one to register a new component: https://github.com/backstage/backstage/blob/e6c52e7e7ec21ceb98cfbaad55c84b3e3683649f/plugins/scaffolder-backend/src/scaffolder/actions/builtin/catalog/fetch.ts#L93

In our setup, we needed to set the credentials: dangerously-allow-unauthenticated temporarily. I've tested it without them again and the requests weren't returning the 401 any more.

This should be fixing #1475, and I followed the idea in backstage/backstage#25096 (comment)

✔️ Checklist

• Added tests for new functionality and regression tests for bug fixes
• Added changeset (run yarn changeset in the root)
• Screenshots of before and after attached (for UI changes)
• Added or updated documentation (if applicable)

[davormilutinovic]

Hi. For some reason your changes are still not working for me.

I have used your fork and there was an error during execution of one template

...There was an issue with your request. Status code: 401 Response body: {"error":{"name":"AuthenticationError","message":"Invalid plugin token; caused by JWTClaimValidationFailed: unexpected "aud" claim value","cause":{"code":"ERR_JWT_CLAIM_VALIDATION_FAILED...

After I reverted from

      const { token } = (await auth?.getPluginRequestToken({
        onBehalfOf: await ctx.getInitiatorCredentials(),
        targetPluginId: 'proxy',
      })) ?? { token: ctx.secrets?.backstageToken };

to

const credentials = await ctx.getInitiatorCredentials(); 

// @ts-expect-error
const token = credentials.token;

It has start working again?

[github-actions]

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