Add Jira issues inputs in addition to product releases

[hluk]

This will allow automated processing of Jira tickets based on specific JQL search query (thought there is currently no useful prerequisite to process the issues).

Additionally, it is possible to implement custom input classes.

Example rule:

- version: 1
  name: "Example Rule"
  inputs:
    - jql: >-
        project = EXAMPLE
        AND label=retasc-test-issue
        AND resolution is EMPTY
  prerequisites:
    - condition: >-
        jira_issue.fields.summary.startswith('[ReTaSC] ')
        and 'automatic' in jira_issue.fields.description
    ...

This change also simplifies managing issues using a unique label: ReTaSC will fetch the unique issue by single identifying label from Jira for each prerequisite. This means there are multiple Jira API queries, but it is much easier to understand then fetching a group of issues using a common label.