Implement ConstantKeyword property

[russcam]

Relates: elastic/elasticsearch#49713

This commit adds the ConstantKeyword property to the client.
Value is exposed as type Object as it can be a string or numeric
value.

[Mpdreamz]

LGTM other than two nitpicks around spacing.

[github-actions]

The backport to 7.7 failed:

The process 'git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.7 7.7
# Navigate to the new working tree
cd .worktrees/backport-7.7
# Create a new branch
git switch --create backport-4593-to-7.7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 38edce2c6553cfac448efaed95b3b0d0fcc7f2f9
# Push it to GitHub
git push --set-upstream origin backport-4593-to-7.7
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.7

Then, create a pull request where the base branch is 7.7 and the compare/head branch is backport-4593-to-7.7.

[github-actions]

The backport to 7.x failed:

The process 'git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.x 7.x
# Navigate to the new working tree
cd .worktrees/backport-7.x
# Create a new branch
git switch --create backport-4593-to-7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 38edce2c6553cfac448efaed95b3b0d0fcc7f2f9
# Push it to GitHub
git push --set-upstream origin backport-4593-to-7.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.x

Then, create a pull request where the base branch is 7.x and the compare/head branch is backport-4593-to-7.x.

[russcam]

manually backported to 7.7 and 7.x