Built-in compression support for layer push/fetch

[vmvarela]

Problem

When pushing artifacts with large layers, users often need to compress data
before pushing. Currently this requires manual implementation of compression
and proper media type handling (e.g., +gzip, +zstd suffixes).

Proposal

Add optional compression support to PushBytes and related functions:

1. PushBytesCompressed() - compress and push with appropriate media type
2. FetchBytesDecompressed() - auto-detect and decompress based on media type
3. Support for gzip and zstd algorithms

Use Cases

• OpenTofu state storage (we implemented this in our ORAS backend)
• Large configuration files
• Log archival
• Any artifact where bandwidth matters

Alternatives Considered

• Manual compression (current approach, duplicated effort)
• External compression before push (loses media type semantics)

Would the maintainers be interested in this feature?

[FeynmanZhou]

@TerryHowe and I thought this is a good proposal but we want volunteers to help move this forward.

Tagging @oras-project/oras-go-maintainers to see if others have opinions on this. I coverted this into a new issue to track #1102