rename Compression struct

[Rich-T-kid]

Which issue does this PR close?

• smaller piece of [10125] [encode path] Minor optimizations to arrow-flight #10137
• see comment [10125] [encode path] Minor optimizations to arrow-flight #10137 (review)

Rationale for this change

Changed Compression context to IPCwritecontext as now its holding onto the fbb and scratch space for buffers.

What changes are included in this PR?

• compression.rs: Renamed CompressionContext → IpcWriteContext. Added scratch: Vec field (unused for now, #[allow(dead_code)]). Added #[deprecated(since = "57.0.0", note = "Use IpcWriteContext instead")] pub type CompressionContext = IpcWriteContext; for backward compatibility.
• writer.rs: Updated all internal type references from CompressionContext -> IpcWriteContext. Renamed all field/parameter/variable occurrences of compression_context -> ipc_write_context. Both names are re-exported from writer so existing code compiles with a deprecation warning rather than a hard error.

Are these changes tested?

n/a

Are there any user-facing changes?

[Rich-T-kid]

cc @alamb
a follow up PR will wire together the scratch buffer and the IPCsink enum together