refactor: propagate time through context values

[knqyf263]

Description

In Trivy, time.Now() has been traditionally handled through two methods:

• Functional Options Pattern (example)
• Global variables (here)

While Functional Options offer advantages in unit testing, they present challenges in integration testing, especially when multiple instances of time.Now() need to be mocked with a consistent, fixed time. This approach often requires repetitive coding in multiple places. Conversely, global variables, though convenient, complicate ensuring thread safety.

To address these issues, this PR introduces a new approach: storing and passing time.Time within the context. As per the documentation guidelines, context values should be used cautiously and are not meant for every situation.

Use context values only for request-scoped data that transits processes and API boundaries, not for passing optional parameters to functions.

However, leveraging context values is a viable solution for data that is integral throughout the entire process flow.

This change allows us to maintain a uniform, fixed time reference throughout a single process while ensuring thread safety. We are open to future improvements and will consider any better methods that may emerge.

This modification enhances our integration testing capabilities. By enabling the fixation of time in tests, we can prevent failures due to End-Of-Life (EOL) related time issues, as was previously encountered in our project, like this.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

All other PRs are failing due to an EOL issue, and this PR should fix it. @chen-keinan @simar7 Can you please take a look?

[knqyf263]

@chen-keinan Addressed your comments. Can you please take another look?

[chen-keinan]

lgtm