Add config declarations for harvest extension

[thorge]

This PR adds config declarations for the CKAN Harvest extension, following the recommended IConfigDeclaration pattern. All relevant configuration options are now explicitly declared, improving validation, normalization, and auto-documentation support.

• The ckan.harvest.timeout option should ideally be declared with declare_int. However, the current implementation in harvest_jobs_run() in update.py checks the value with if timeout:, and expects None or False as default. Using declare_int would require adjustments to the timeout handling logic to safely support integer defaults.

Fixes #564

[thorge]

I changed the default value of ckan.harvest.mq.password to None to avoid breaking passwordless Redis connections. When no password is provided, the extension expects the value for Redis to be None, whereas for RabbitMQ it assumes a default password of guest. The current implementation conflates these differing assumptions, so this part of the code may need rework to correctly support a common default across both Redis and RabbitMQ.

Possible solutions include splitting the configuration variables per backend, or introducing more explicit conditional handling based on a shared default value. Should we address this as part of this PR?