As a user, I can export Xray issues into JSON

As system, I must provide a REST API endpoint to export all Test, Preconditions, Test Sets, Test Executions (excluding test runs) and Test Plans, so that it can be used to migrate data from JIRA Cloud to JIRA server.

The only possible parameter to the endpoint is projectIds which is an array of project ids to export.

The generated document must contain an array of Test documents with all Test Specification fields including:

• Test Id
• Test Issue Id
• Project Id
• Test Type (name)
• Test Type (Id)
• Test Steps
• Generic Definition
• Gherkin Definition
• Pre-Condition Issue Ids

For Preconditions, the following fields should be exported:

• Steps/Definition/Scenario

The JSON format shall be compatible with Jira's Server JSON importer (https://confluence.atlassian.com/adminjiraserver/importing-data-from-json-938847609.html) so it can be used with it.