Execute CI job via Step Runner service - Docker Executor

Problem

Customers want to run steps using the Docker executor.

Proposal

Execute a CI job via the Step Runner service for executordocker according to https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/runner-integration.html#docker

This includes:

• Finding a good place to intercept "regular" script execution and replace it with delegation to steps.
• Creating RunRequest from the Job request (#37413 (closed)).
• Creating a connection function that will (ab)use the docker exec protocol and allow sending gRPC requests over it (See !4555 (closed) for working example for executordocker).
• Using the step service client library to manage the entire execution lifecycle.
• Step Runner service logs should be streamed to the CI job trace log.
• Step Runner StepResults can be /dev/null for now.

Notes:

• Assume the step-runner binary exists in the target host, and the steps-runner service is running, listening on a local socket (see https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/runner-integration.html#non-goals).
• Assume the step-runner binary has a proxy command.