Add dsn to error tracking client key

What does this MR do?

Generate a DSN (special kind of url) that can be used by sentry client to submit exceptions to GitLab backend collector.

Before that change I manually assembled such URL. Now I can simply query the sentry_dsn method in rails console.

This method will be used in follow-up merge requests to render to user so they can copy-paste it into sentry sdk.

Issue #329596 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)