Add composable function `useAuth` for setup script

[denysoblohin-okta]

• Adds composable useAuth() to access the oktaAuth instance in setup script
• Added unit test
• Added change-log for new version 5.4.0

Copy of https://github.com/okta/okta-vue/pull/102 Resolves #101 Internal ref: https://oktainc.atlassian.net/browse/OKTA-516507

[Ancient-Dragon]

@denysoblohin-okta any chance of a release soon?

[toneill818]

Is there a release date for 5.4?

[jaredperreault-okta]

5.4.0 has been released