[Buildkit] Refactoring builds function to interact with buildkit sdk

### What is the problem you're trying to solve

Currently nerdctl forks a process to run buildctl for the builds. This creates an issue for finch-daemon to pass in credentials received as part of the API request to pass onto buildctl. One way is to write it to the docker/config.json but that is not inherently secured.

### Describe the solution you'd like

Options:
1. Use buildkit sdk for the build functionalities in nerdctl.
2. Add an username, password option to buildctl to take in as argument.

@AkihiroSuda any suggestions which is a preferable option. Personally option 1 seems to be a better solution as it moves away from forking a process from the daemon along with handling the credentials. Let me know your thoughts.

### Additional context

_No response_