call ApplyTFConversions in Update function from terraform plugin sdk external client

[digna-ionos]

Not calling ApplyTFConversions in Udate function of tf plugin sdk external client was causing unmarshalling errors for singleton lists.

Description of your changes

add ApplyTFConversions call in Update function, after marshalling json state and before setting the observation.

Fixes https://github.com/crossplane/upjet/issues/438

I have:

• [x] Read and followed Upjet's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

There are already tests in place for Update function.

[mergenci]

@digna-ionos, Can you share details of your use case that required this change? While I agree that this change is likely to be required, the fact that we haven't hit an issue for so long makes me doubt — and the impact of this change doesn't help relieve my doubt 🙂 I would be happy if you could provide more details. If you could also provide an example manifest that fails before the change and passes after, it would even be better.

[digna-ionos]

@mergenci I've prepared two tests to make it simpler to understand and play with it if you need. https://github.com/crossplane/upjet/commit/9a5726e09448cbb026690a7217b87764dd4e0297#diff-88b92dceed2024799f8225f8778bfc0c159fb2cc590311563721615aa2367ec9