OKTA-1218860: Add App user function in OEL

[janugeethakumari-okta]

Description:

• What's changed?
• Is this PR related to a Monolith release?

Resolves:

• OKTA-1218860

Netlify Preview Link:

[okta-prod-github-app]

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Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/reference/okta-expression-language-in-identity-engine/index.md	81	91	73	85

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