Add fastly.decompressGzip option for automatic decompression

[trieloff]

Background

Per adobe/helix-deploy-plugin-edge#77 and comment, @adobe/fetch should support the fastly: { decompressGzip: true } option to align Fastly and Cloudflare's fetch behavior.

Proposal

Add support for the Fastly-specific decompressGzip option within a fastly options object:

import { fetch } from '@adobe/fetch';

const response = await fetch(url, {
  fastly: {
    decompressGzip: true
  }
});

Behavior

• Fastly: Pass through to native fastly.decompressGzip option
• Cloudflare: No-op (Cloudflare automatically handles decompression)
• Node.js: Map to existing compress/decode options

Context

Cloudflare Workers automatically decompress gzip responses. Fastly requires explicit opt-in via decompressGzip. Supporting this option in @adobe/fetch allows writing portable code that works identically across platforms.

Implementation Notes

Since @adobe/fetch already handles compression transparently via the compress and decode options (defaults to true), this would primarily be:

1. Accept fastly.decompressGzip in options
2. Map to platform-appropriate behavior
3. Document as the cross-platform way to control decompression

[trieloff]

@adobe/fetch uses the { decompress: true } option. Default is true. So, unless decompress is false, the fastly.decompressGzip option should be set in the wrapper.