Create models for issue trackers data

What does this MR do?

We want to move data from issue trackers serialized properties (Service model) to a separate table.

This MR creates 2 new models, one for data of jira and one for general issue trackers.

Reading and writing data will be a follow-up in order to keep the scope small and make MRs easier to review/discuss.

Issue

https://gitlab.com/gitlab-org/gitlab-ce/issues/63081

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Performance and testing

• we have to iterate over all issue trackers in the migration and create records for them. As we need to encrypt the fields we can't do simple bulk inserts

If cross-browser testing is not required, please remove the relevant item, or mark it as not needed: [-] -->

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team