REST: Add ETag and conditional GET support for loadView

### Proposed Change

The `loadTable` endpoint (`GET /v1/{prefix}/namespaces/{namespace}/tables/{table}`) supports conditional requests via the `If-None-Match` request header and returns an `ETag` response header (spec lines 965–977). When the table metadata hasn't changed, the server returns `304 Not Modified`, saving serialization and transfer costs. The Java reference client (`RESTSessionCatalog`) implements this with a Caffeine-backed `RESTTableCache` that stores `(sessionId, TableIdentifier) → (table, eTag)` mappings.

However, the `loadView` endpoint (`GET /v1/{prefix}/namespaces/{namespace}/views/{view}`) has **no equivalent support**:

1. **Spec gap:** The `loadView` operation ([rest-catalog-open-api.yaml, line ~1580](https://github.com/apache/iceberg/blob/main/open-api/rest-catalog-open-api.yaml#L1580)) defines no `If-None-Match` header parameter and no `304` response. The `LoadViewResponse` response object (line ~4953) has no `etag` header, whereas `LoadTableResponse` (line ~4943) and `CommitTableResponse` (line ~4974) both include one.

2. **Client gap:** `RESTSessionCatalog.loadView()` (line ~1427 in RESTSessionCatalog.java) always issues a full `GET` with no headers and no caching. There is no `RESTViewCache` equivalent to the existing `RESTTableCache`.

Views are increasingly used for cross-engine SQL compatibility layers and can have substantial metadata (multiple version entries, representations, schema history). Workloads that repeatedly resolve or validate views pay the full round-trip cost every time.

**Proposed spec changes:**

- Add an optional `If-None-Match` header parameter to the `loadView` GET operation, matching the existing pattern on `loadTable`.
- Add a `304 Not Modified` response to the `loadView` GET operation.
- Add an `etag` response header to `LoadViewResponse`, matching `LoadTableResponse` and `CommitTableResponse`.
- Optionally add an `etag` response header to the `replaceView` (`POST`) response, matching `commitTable`, so the cache can be primed on writes.

**Backward compatibility:**

- `If-None-Match` is an optional header; servers that don't support it simply ignore it and return `200` as today.
- Clients that don't send the header see no behavioral change.

**Willingness to contribute**

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

### Proposal document

_No response_

### Specifications

- [ ] Table
- [x] View
- [x] REST
- [ ] Puffin
- [ ] Encryption
- [ ] Other

Compare the loadView operation around line 1580 in rest-catalog-open-api.yaml with loadTable, including LoadViewResponse around line 4953. Then inspect RESTSessionCatalog.loadView() around line 1427 and the existing RESTTableCache pattern. Done means loadView supports If-None-Match, 304 responses, and ETag headers consistently with loadTable; consider the replaceView response separately.

Written by the indexing model from the issue text.