feat: secret attachment modal

[thaorell]

Description:

• Created a secrets attachment modal, which allows users to mount existing Secrets from the /secrets/${namespace} endpoint to the creation flow
• Refactored secret create modal to its own file

closes: #660
depends on: #671

[google-oss-prow]

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[thaorell]

Screen.Recording.2025-11-06.at.1.41.31.PM.mov