[7.x] [Form lib] Correctly add field to form on component mount (#75796)

[sebelga]

Backports the following commits to 7.x:

• [Form lib] Correctly add field to form on component mount ([Form lib] Correctly add field to form on component mount #75796)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 8a980f5

Build metrics

async chunks size

id	value	diff	baseline
ingestPipelines	705.5KB	+22.0B	705.5KB
securitySolution	9.5MB	+467.0B	9.5MB
total		+489.0B

page load bundle size

id	value	diff	baseline
esUiShared	990.6KB	+1.4KB	989.2KB
indexManagement	737.2KB	+409.0B	736.8KB
total		+1.8KB

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