Support multisearch command in calcite

[ahkcs]

Description

This PR introduces the multisearch command, a new PPL command that allows combining results from multiple search subsearches into a single unified result set.

Resolves: #4348

Key Features:

• Combines results from 2+ search subsearches with different filtering criteria
• Supports ALL PPL commands within subsearches (where, eval, fields, stats, sort, dedup, etc.)
• Enables timestamp-based result interleaving for time-series data analysis
• Uses efficient UNION ALL + ORDER BY architecture for optimal performance

Example Usage:

-- Basic age group analysis
source=accounts | multisearch
  [search source=accounts | where age < 30 | eval age_group = "young"]
  [search source=accounts | where age >= 30 | eval age_group = "adult"]
| stats count by age_group

-- Success rate monitoring pattern
source=logs | multisearch
  [search source=logs | where status="success" | eval result="success"]
  [search source=logs | where status!="success" | eval result="total"]
| stats count(eval(result="success")) as success_count, count() as total_count

[dai-chen]

np: do we really need so many test data to demonstrate the command usage in documentation?

[ahkcs]

I was thinking we can reuse this dataset for future doctest for timestamp

[ahkcs]

I updated to trim the dataset to 20 records

[dai-chen]

Do we add and benchmark Big5 query for each new command like this?

[ahkcs]

Do we add and benchmark Big5 query for each new command like this?

I think we can add IT tests to benchmark Big5 queries like this, I'll open a separate PR for this:
#4163

[ahkcs]

Do we add and benchmark Big5 query for each new command like this?

For Multisearch command, I think the result will be exactly the same as Append command as they share the same implementation

[opensearch-trigger-bot]

The backport to 2.19-dev failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/sql/backport-2.19-dev 2.19-dev
# Navigate to the new working tree
pushd ../.worktrees/sql/backport-2.19-dev
# Create a new branch
git switch --create backport/backport-4332-to-2.19-dev
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ca5a5bd6ee36ecdbbb2d3dc7a4b73ba28ecafa95
# Push it to GitHub
git push --set-upstream origin backport/backport-4332-to-2.19-dev
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/sql/backport-2.19-dev

Then, create a pull request where the base branch is 2.19-dev and the compare/head branch is backport/backport-4332-to-2.19-dev.