Add teleport debug readyz command

[wethreetrees]

Purpose

Adds a command to the teleport cli to query the /readyz endpoint to accommodate restricted environments, e.g. curl unavailable.

resolves #62176

Notes

Exits with code 0 and prints to stdout on success
Exits with non-zero code and prints readyz status or associated error to stderr on failure

Manual Testing

The below snippets will work for a quick validation of both good and bad cases. There seemed to be little benefit to automated tests, as the logic is very simple and we'd be mocking the service side.

Sample testing output can be found here: #62491 (comment)

1. Start teleport and run the new teleport debug readyz command before it's ready.

# hacky, but should hit before the service is ready
./build/teleport start &
teleport_pid=$!
sleep 0.5
./build/teleport debug readyz

2. Wait a few seconds for the service to be ready and then run again to see the "success" case.

./build/teleport debug readyz

3. Cleanup the teleport process.

kill $teleport_pid

changelog: Added teleport debug readyz command

[wethreetrees]

Manual Testing Output

Bad case (not ready)

Good case (ready)

[backport-bot-workflows]

@wethreetrees See the table below for backport results.

Branch	Result
branch/v18	Create PR

[taraspos]

This is super useful for health-checking Teleport Agents running on ECS Fargate, thanks!