[EuiFlyout] Memoize various internals to reduce rerenders

[cee-chen]

Summary

The first commit is the primary one needed to resolve elastic/kibana#168164, the other following commits are just extra nice-to-haves.

Here's what I think is happening: the unmemoized focusTrapProps is causing <EuiFocusTrap> the underlying focus trap library to re-mount fully when it shouldn't, which then causes the focus issues because the focus trap will auto-focus the flyout wrapper on mount.

The remount is occurring because Security's flyouts are wrapped in a <Formik> lib that, on validation, cascades the state update down to the wrapped EuiFlyout and rerenders it.

QA

Fix testing

• Go to https://cee-chen-branch-main.kbndev.co/app/management/security/api_keys
• Log in with dev/local elastic credentials
• Click the "Create API key" button
• Once the flyout opens, click into the Name field and then press the tab key
• Confirm an invalid error shows up
• Confirm that you can continue to tab past the Name field and through the rest of the flyout, but that focus correctly traps and cycles back to the fixed headers and then back into the flyout

Regression testing

• Go to https://eui.elastic.co/pr_7259/#/layout/flyout
• Confirm all flyouts on the page behave as before, particularly closing (click + Escape to close)
• Go to https://eui.elastic.co/pr_7259/#/layout/flyout#understanding-max-width
• Confirm all flyouts have the expected inline widths & max width styles / same as production
• Go to https://eui.elastic.co/pr_7259/storybook/?path=/story/euiflyout--playground
• Confirm adding style, size (number) and maxWidth (number) controls concatenates all the inline styles as expected

General checklist

• Browser QA - N/A, above QA should be sufficient
• Docs site QA - N/A
• Code quality checklist - N/A, should be sufficient for regressions if existing tests pass
• Release checklist
  • A changelog entry exists and is marked appropriately.
    - [ ] If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist - N/A

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7259/
• Storybook: https://eui.elastic.co/pr_7259/storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 0cc6f40

[1Copenut]

Did a quick keyboard and mouse QA on the four evergreen browsers. All flyouts were behaving as in production, saw no regressions.

[cee-chen]

Thanks Trevor!

@SiddharthMantri - this will be be in EUI's Monday release, and should reach Kibana main sometime by end of next week (fingers crossed).