fix: use subscription fields as string

[Xillians]

TL;DR

according to tripletex api, the /event/subscirption expects the fieldto be a string rather than an array

What's changed

• turned subscription events endpoint to expect string

Why

This is to make the subscription use the expected input to avoid getting 422 errors.

[braaar]

It appears to me that the fields value is turned into a comma-separated string in the actuall call

[Xillians]

It appears to me that the fields value is turned into a comma-separated string in the actuall call

That was the intention. The documentation for event/subscription was added to the description along with the explanation

[braaar]

This PR is just circumventing the toString method by expecting the SDK user to create a comma separated string. I don't see the point of this change.