feat(image): introduce WithCredentials* to select how pull credentials are retrieved

[ndeloof]

What does this PR do?

introduce a generic credentialsFn to retrieve credentials for pull operation
offers WithCredentialsFromConfig to get credentials from docker/cli config (current mechanism)
this makes it possible to plug an alternative func that can resolve credentials from a TBD mechanism use by the docker/cli after cli/config cleanup (see discussion on docker/cli#6210), as storing credentials in config is not the desired way.

This mkes legacyadapter to convert docker/cli Config <-> go-sdk/config useless as (AFAICT) it is only used by Pull operation to retrieve username/password. Here we model this requirement as a minimal lookup method.

[Copilot]

Pull Request Overview

This PR introduces a generic credentials function mechanism to make the credential retrieval process pluggable for Docker image pull operations. The change replaces the hardcoded Docker CLI config credential lookup with a configurable function, enabling alternative credential resolution mechanisms.

• Adds a configurable credentialsFn field to pullOptions struct for credential retrieval
• Introduces WithCredentialsFn option to set custom credential functions
• Moves existing config-based credential logic to WithCredentialsFromConfig function
• Sets WithCredentialsFromConfig as the default when no custom credential function is provided

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
image/pull.go	Replaces direct config-based auth logic with pluggable credential function calls
image/options.go	Adds credential function field and new option functions for configuration

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[mdelapenya]

I added some unit tests, other than that LGTM!! 🙇

Will merge it as soon as the CI passes.