Add JIRA change ticket ID to chain of custody report

Problem to solve

Compliance-minded organizations have strict auditing requirements that force them to document all changes they make to production environments. Part of this process involves the use of a change control system such as JIRA or ServiceNow where change tickets are created to document the business case for the change. Since these systems are external to GitLab, there's currently no way to formally establish a relationship between a GitLab MR and the required change ticket.

Intended users

• Cameron (Compliance Manager)
• Parker (Product Manager)
• Delaney (Development Team Lead)
• Sasha (Software Developer)
• Sidney (Systems Administrator)
• Rachel (Release Manager)

User experience goal

When downloading the chain of custody report, the report should contain any related change ticket linked to each respective MR.

Proposal

Add an additional column to the chain of custody report for Change Ticket, which should list any ticket ID added to an MR by way of #235616 (closed).

Merge Commit	Author	Merge Request	Merged By	Pipeline	Group	Project	Approver(s)	Change Ticket
SHA	Bugs	!45	Bugs	pipeline_id	My-Group	awesome-project	Daffy Duck	TICKET-1234
SHA	Lola	!45	Lola	pipeline_id	My-Group	awesome-project	Marvin	TICKET-1234
SHA	Daffy	!46	Daffy	pipeline_id	My-Group	awesome-project	Lola	TICKET-567
SHA	Yosemite	!46	Yosemite	pipeline_id	My-Group	awesome-project	Marvin	TICKET-567
SHA	Marvin	!46	Marvin	pipeline_id	My-Group	awesome-project	Lola	TICKET-567
SHA	Lola	!47	Lola	pipeline_id	My-Group	awesome-project	Bugs	TICKET-8910

Implementation plan

1. Add a new Change Ticket column to MergeCommits::ExportCsvService#header_to_value_hash with the value coming from the #235616 (closed) implementation
2. Update ee/spec/services/merge_commits/export_csv_service_spec.rb and other related tests to test the new column

Further details