MDEV-26312: Record iter methods for JOIN_CACHE_BNL

[LinuxJedi]

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This commit creates new public methods to iterate the join cache. This is useful for engine condition pushdown in remote storage engines. In addition to push the condition, we can also push data to match rows during JOIN. This avoids full table scan and hence full table read over the network.

Commit copied from #2176 and rebased to 11.3 to eliminate the merge commit.

• [x] The Jira issue number for this PR is: MDEV-26312

Basing the PR against the correct MariaDB version

• [x] This is a new feature and the PR is based against the latest MariaDB development branch.
• [ ] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.