Add toString() implementations

[swallez]

Adds toString() implementations to all classes:

• for requests it outputs the request method, URL path & parameters, body as JSON
• for all other classes it outputs the JSON representation of the object

For classes containing user-defined objects like an indexing request or a search response, these object's toString() result is output as a JSON string value. The reason for that is because in toString() we don't have access to the JsonpMapper that defines how to serialize user-defined classes.

Fixes #97, part of #101

[github-actions]

The backport to 7.17 failed:

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

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-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-269-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 8e4db1dea8897d3acfca72ab501be7c0bca91e5b
# Push it to GitHub
git push --set-upstream origin backport-269-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-269-to-7.17.