Introduce hash_code for VSA stages and events

What does this MR do?

Related issue: #335291 (closed)

Not user-facing change. This MR adds the hash_code method to Stage Events and Stages. This is needed to reduce the amount of timestamp (stage data) data we collect within the new VSA database tables. The hash codes are currently not persisted, this will happen in a follow-up MR.

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.

Related to #335291 (closed)