cluster_link/model: change default max partition bytes

Prior to this commit the default max partition bytes was 1MiB. The
default min fetch bytes was 5MiB. And the max wait ms was 500ms.

In cases where only one partition on the shard was being produced to
these defaults limited our consumer throughput to 2MiB/s. This is since
we'd only be able to read 1MiB every 500ms from the one partition with
new data.

Hence this commit sets max partition bytes to equal min fetch bytes to
prevent this issue.

Author: ballard26

src/v/cluster_link/model/types.h

@@ -276,7 +276,7 @@ struct connection_config
     // fetch response.
     std::optional<int32_t> fetch_partition_max_bytes;
     // Default maximum number of bytes to fetch per partition
-    static constexpr auto default_fetch_partition_max_bytes = 1_MiB;
+    static constexpr auto default_fetch_partition_max_bytes = 5_MiB;
     // Default maximum number of bytes to fetch (20MiB)
     static constexpr auto fetch_max_bytes_default = 20 * 1024 * 1024;