Support customizing the base image for python apps using UV

[davidfowl]

Today we're generating a docker file using uv for the multistage build and python for the base image. We should support overriding this via annotations.

See

aspire/src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs

Line 391 in dcae024

.From($"ghcr.io/astral-sh/uv:python{pythonVersion}-bookworm-slim", "builder")

and

aspire/src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs

Line 414 in dcae024

.From($"python:{pythonVersion}-slim-bookworm", "app")

[eerhardt]

Implemented with #12566.