feat: support non-JSON content types in write_attachments/read_attachments

[mother-6000]

Context

The server (reductstore/reductstore#1371) now supports storing and retrieving schema/meta attachments in arbitrary formats (not just JSON). The storage uses the record's content_type to determine format. Existing JSON workflows are unchanged.

Required Changes

write_attachments

Add an optional content_type parameter (default: "application/json"):

• If content_type is JSON-like (application/json, text/json, *+json): serialize values with the language's JSON encoder (current behavior, no change)
• Otherwise: accept raw bytes and pass them directly as the record payload with the given content_type

read_attachments

Branch on the record's content_type response header:

• If JSON-like: deserialize as today (no change)
• Otherwise: return raw bytes along with the content_type string

Acceptance Criteria

• write_attachments accepts optional content_type parameter
• Non-JSON payloads are written as raw bytes with correct content_type header
• read_attachments returns raw bytes + content_type for non-JSON records
• Existing JSON-only callers continue working without changes (default behavior preserved)
• Unit tests cover at least one non-JSON format (e.g. application/x-protobuf)
• No breaking changes to existing public API

Notes

• The underlying write mechanism already supports arbitrary content_type — this is just plumbing it through the attachment helper methods
• The $meta entry write path uses begin_write with an explicit content_type, so no server API changes are needed
• Parent issue: Feature: support schema attachments in formats beyond JSON reductstore#1371

Parent issue: reductstore/reductstore#1371