Add an _exists_ check to document level monitor queries

[jowg-amazon]

Issue #, if available: #854

Description of changes: Related to https://github.com/opensearch-project/security-analytics/pull/852

Checks if _exists_ is present in the query. If it is, then replace the value with the field name and the correctly appended index name and monitor id.

CheckList:

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[jowg-amazon]

@eirsep Opensearch allows null to be passed in as a value so a not null check would be different than an exists check. We want to check whether or not the field even exists in the document, not if the field is null.

[jowg-amazon]

shouldn't there be : between exists and ?

@eirsep Removed the _exists_ flag so it parses to find wherever the query is _exists_: <field name> and transforms the

[opensearch-trigger-bot[bot]]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

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/alerting/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/alerting/backport-2.x
# Create a new branch
git switch --create backport-1425-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 afa4f5d131a7075593c56efb61ba95d01ad82ebe
# Push it to GitHub
git push --set-upstream origin backport-1425-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/alerting/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport-1425-to-2.x.

[opensearch-trigger-bot[bot]]

The backport to 2.11 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/alerting/backport-2.11 2.11
# Navigate to the new working tree
pushd ../.worktrees/alerting/backport-2.11
# Create a new branch
git switch --create backport-1425-to-2.11
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 afa4f5d131a7075593c56efb61ba95d01ad82ebe
# Push it to GitHub
git push --set-upstream origin backport-1425-to-2.11
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/alerting/backport-2.11

Then, create a pull request where the base branch is 2.11 and the compare/head branch is backport-1425-to-2.11.