Support Content-Encoding header for upload routes

[Athaxv]

The Supabase Storage REST API (/object/* routes) currently ignores the Content-Encoding header during uploads. This prevents clients from uploading pre-compressed objects (e.g. gzip) and having them transparently decompressed by browsers on download.

The S3-compatible API (/s3/* routes) already supports Content-Encoding. This request is to bring parity to the REST API.

This feature is required to support use cases like:

Uploading large gzip-compressed files from Edge Functions
Reducing bandwidth and storage costs
Allowing browsers to automatically decompress responses

There is an existing client-side PR supabase-js that adds support for sending Content-Encoding, but it cannot work end-to-end until server support is added.