OpenAPI: Recommend a standard User-Agent format for REST clients

[rahulsmahadev]

What

Adds a recommended, consistent User-Agent format to the REST spec so clients identify themselves the same way, and documents that client identification is optional and informational only.

Spec-only proposal

Why

Clients identify themselves inconsistently today, and the most-deployed one doesn't by default:

Client	User-Agent	X-Client-Version
iceberg-java	none by default (opt-in rest.client.user-agent)	Apache Iceberg <ver> (commit <sha>) — library version
pyiceberg	PyIceberg/<ver>	PyIceberg <ver> — library version
iceberg-rust	iceberg-rs/<ver>	0.14.1 — REST spec version
iceberg-go	GoIceberg/<ver>	REST spec version

A catalog operator can't reliably tell what's calling: the Java client sends no User-Agent unless configured, the naming differs across clients, and X-Client-Version carries the library version in two clients and the spec version in the other two. None of it is described in the spec.

A single recommended format gives operators usable telemetry for observability, debugging, and support, and an Iceberg library token that's always present regardless of the embedding engine.

Proposal

Use the standard User-Agent header (RFC 7231): whitespace-separated product/version tokens, most-specific first (engine → integration → Iceberg library → runtime), with a parenthesized comment as an open extension point.

Spark/4.0.0 iceberg-spark/1.9.0 iceberg-java/1.9.0 (scala/2.13.16)

• The Iceberg library token (iceberg-java, pyiceberg, iceberg-rust, iceberg-go) is the piece every client can supply and should always be present.
• Optional and informational: servers must not reject on missing/malformed, must not use it for auth or trust decisions, and it is not capability negotiation — that's the versioned-endpoints discussion from Core, OpenApi: Add X-Iceberg-Client-Capabilities header #16394, kept deliberately separate.

Scope / follow-ups

This PR is the spec recommendation. If the format lands, client conformance follows as separate PRs:

• iceberg-java: give rest.client.user-agent a default library token and a way for integrations (Spark/Flink/…) to contribute engine tokens.
• pyiceberg / iceberg-rust / iceberg-go: emit the same format.