Fix `Jira` constant autoloading problem in development

What does this MR do?

In !61743 (merged) we renamed a model file from JiraService to Integrations::Jira.

Subsequently, in development mode, Rails would error when we called the service files that are in the top-level ::Jira namespace, rather than load them:

• app/services/jira
• ee/app/services/jira

This MR fixes this issue by adding an empty Jira module #331756 (comment 583983122).

QA steps

Compare this branch to master by starting a Rails console on each branch and typing:

::Jira::Requests::Projects::ListService

On master currently, this will error with:

LoadError: Unable to autoload constant Jira, expected /Users/luke/Sites/gdk-ee/gitlab/app/models/integrations/jira.rb to define it

Whereas it will load without problems on this branch.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #331756 (closed)