feat: include access header (x-vercel-blob-access)

[pappabully-maker]

This patch adds an HTTP header x-vercel-blob-access when creating or copying blobs so PHP SDK calls can indicate access: 'private' or access: 'public' to the Vercel Blob API.

Background:

• The upstream PHP client currently encodes options as x-* headers but does not include the access option as a header.
• Vercel Blob server-side validation may require access be communicated (or present as query param) for certain store configurations.

Files included:

• src/Client.php (modified): adds x-vercel-blob-access in getHeadersForCommonCreateBlobOptions() when CommonCreateBlobOptions->access is set.

How to test locally:

1. Install dependencies and run unit tests for the package (if present).
2. Use the SDK to call put() with CommonCreateBlobOptions('private', ...) against a private store and assert the API accepts the request.

Suggested PR title: "feat: include access header (x-vercel-blob-access) for CommonCreateBlobOptions"

Notes:

• This patch was generated from an integration change in a consuming project. Please review for style and tests before merging.