Add structured logging to ImageBuilder

Adding structured logging to ImageBuilder would allow us to analyze log outputs and operate on log data using `jq` or other tools.

I propose that we split ImageBuilder's logging into two components:

- High-level console logging that is concise (fits on ~one terminal screen) and easily parsable by humans and LLMs.
- Detailed structured logging in json that is output to a file and published as a pipeline artifact. One log per ImageBuilder invocation. Put each log file into one folder and upload one log artifact per pipeline job.

Microsoft.Extensions.Logging does not have file logging support natively. See https://github.com/dotnet/runtime/issues/59322. There are a couple libraries that can provide this functionality through M.E.Logging.Abstractions (using ILogger) - both are popular and in active development:

- [Serilog](https://www.nuget.org/packages/Serilog) (~2B downloads)
- [NLog](https://www.nuget.org/packages/NLog) (~500M downloads)

[NLog version 6](https://nlog-project.org/2025/06/21/nlog-6-0-released.html) fully supports Native AOT/trimming so I lean towards that one.

Inizia individuando il punto di ingresso di ImageBuilder e verificando l’utilizzo attuale di Microsoft.Extensions.Logging. Esamina quindi come i job della pipeline pubblicano gli artefatti e confronta gli approcci proposti con Serilog e NLog. Il lavoro è completato quando sono presenti un output conciso nella console, un log JSON strutturato per invocazione e un artefatto di log caricato per ogni job della pipeline.

Scritto dal modello di indicizzazione a partire dal testo della issue.