Expose webhook data for ProjectMember via its own data-builder class

What does this MR do?

Related issue #290290 (closed)

This refactor helps to get all event data required for firing project member related webhooks from its own data-builder class, namely Gitlab::HookData::ProjectMemberBuilder so that this information is not private to SystemHooksService anymore.

No changelog added as this is only a refactor.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
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