fix: add prop for custom loading text

[seanes]

Description

Related Issue(s)

• #{issue number}

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[allinox]

Thanks for this quick fix for the loading text on the button!
Looks great! 😄