[APM] Add tests for agent key management

[gbamparop]

Agent key management functionality has been added to the APM settings by #119543 and #120373.

• API tests:

Add tests for the following endpoints:

• GET /internal/apm/agent_keys Adds API tests for APM agent keys #125302
• GET /internal/apm/agent_keys/privileges
• POST /internal/apm/api_key/invalidate Adds API tests for APM agent keys #125302
• POST /api/apm/agent_keys Adds API tests for APM agent keys #125302

Unit tests

Add tests for agent key components:

• Different states (e.g. no data, API keys not enabled, permission denied, loading)
• Agent keys table
• Create agent key flyout

e2e tests

• Add test for getting, creating and deleting agent keys (similar to ftr_e2e/cypress/integration/power_user/settings/custom_links.spec.ts)

Storybook

Add stories: #125407

• Different states (no data, API keys not enabled, permission denied, loading)
• Agent keys table
• Create agent key flyout

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[dannycroft]

@gbamparop to include in next test plan?

[gbamparop]

@gbamparop to include in next test plan?

Yes please, I've added some information to the description.