Add base time option

[m-mizutani]

This pull request introduces several enhancements and bug fixes to the hatchery package, focusing on adding timestamp functionality, improving logging, and modifying configurations for the Slack source. The most important changes include adding a base time flag, updating log messages to include the base time, and modifying the limit configuration for Slack API requests.

Enhancements to timestamp functionality:

• cli.go: Added a base-time flag to the CLI for specifying a base time, with the default being the current time. This flag is injected into the context if provided. [1] [2] [3] [4]

Improvements to logging:

• source/one_password/one_password.go: Updated the log message in the New function to include the base_time value.
• source/slack/slack.go: Updated the log message in the New function to include the base_time value.

Modifications to Slack source configuration:

• source/slack/slack.go: Increased the default Limit value from 100 to 1000 and updated the WithLimit function to handle zero or negative values by excluding the limit parameter from the request. [1] [2] [3]