[securitySolutions] resolve most @kbn/imports/no_boundary_crossing violations

[spalger]

This PR addresses most of the @kbn/imports/no_boundary_crossing violations which were highlighted by #136911.

These changes fall into two categories:

1. mocks were being re-exported from index.ts files which included them in the bundled browser code or server code unnecessarily

   to fix these the re-exports were just removed and the imports using these were updated to import from .../mocks instead

2. mocks were being used to demonstrate specific events in the "Customize Event Renderers" view, which meant that test code was being included in browser bundles

   to fix these the mocks necessary in the browser were moved to a new "demo_data" directory, and where then re-exported from the "mocks" so that copious test imports could be left untouched.

The bundle metrics highlight the benefit of these changes, removing over 200 modules from the browser bundle and hundreds of KB from the async chunks. Additionally, these changes will enable us to maintain separate dependency trees for the tests in security solutions automatically, leading us to better support for incremental tasks in future versions of the packaging system.

There is one remaining issue that I need to brainstorm with someone (likely @oatkiller, based on his work in #75799) but we can take care of that in a separate step.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 94abd18

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
securitySolution	3251	3034	-217

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
securitySolution	5.6MB	5.4MB	-246.7KB

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
securitySolution	254.7KB	254.5KB	-177.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
securitySolution	419	400	-19

Total ESLint disabled count

id	before	after	diff
securitySolution	495	476	-19

History

• 💚 Build #63709 succeeded d93fde0
• 💔 Build #63692 failed 128a951
• 💛 Build #63286 was flaky 276198b
• 💚 Build #62827 succeeded a49e703
• 💚 Build #62515 succeeded e468c93
• 💔 Build #62469 failed ffa2a9628f7142f4f6298d6e3e19a0f7b5e6b582

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[paul-tavares]

Thanks for doing this @spalger . Looks good from a security solution Onboarding and Lifecycle Management team