Improve InstanceDiff to output JSON

### What problem are you facing?

When diagnosing issues like continual updates in a resource, we often enable debugging to review the InstanceDiff.

However, when extracting this structure, it is not a JSON object, but the TF struct embedded as a string under the key `instanceDiff`. This makes it more difficult to determine what fields are causing the update activity.

```json
{
"uid": "465f2136-cc37-4814-a082-13b0a4969928",
"name": "example",
"gvk": "memorydb.aws.upbound.io/v1beta1, Kind=User",
"instanceDiff": "*terraform.InstanceDiff{mu:sync.Mutex{state:0, sema:0x0}, Attributes:map[string]*terraform.ResourceAttrDiff{\"access_string\":*terraform.ResourceAttrDiff{Old:\"\", New:\"on ~* &* +@all\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"arn\":*terraform.ResourceAttrDiff{Old:\"\", New:\"\", NewComputed:true, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"authentication_mode.#\":*terraform.ResourceAttrDiff{Old:\"0\", New:\"1\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"authentication_mode.0.password_count\":*terraform.ResourceAttrDiff{Old:\"\", New:\"\", NewComputed:true, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"authentication_mode.0.type\":*terraform.ResourceAttrDiff{Old:\"\", New:\"password\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"minimum_engine_version\":*terraform.ResourceAttrDiff{Old:\"\", New:\"\", NewComputed:true, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags.%\":*terraform.ResourceAttrDiff{Old:\"0\", New:\"3\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags.crossplane-kind\":*terraform.ResourceAttrDiff{Old:\"\", New:\"user.memorydb.aws.upbound.io\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags.crossplane-name\":*terraform.ResourceAttrDiff{Old:\"\", New:\"example\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags.crossplane-providerconfig\":*terraform.ResourceAttrDiff{Old:\"\", New:\"default\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags_all.%\":*terraform.ResourceAttrDiff{Old:\"0\", New:\"3\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags_all.crossplane-kind\":*terraform.ResourceAttrDiff{Old:\"\", New:\"user.memorydb.aws.upbound.io\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags_all.crossplane-name\":*terraform.ResourceAttrDiff{Old:\"\", New:\"example\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"tags_all.crossplane-providerconfig\":*terraform.ResourceAttrDiff{Old:\"\", New:\"default\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:false, Sensitive:false, Type:0x0}, \"user_name\":*terraform.ResourceAttrDiff{Old:\"\", New:\"example\", NewComputed:false, NewRemoved:false, NewExtra:interface {}(nil), RequiresNew:true, Sensitive:false, Type:0x0}}, Destroy:false, DestroyDeposed:false, DestroyTainted:false, RawConfig:cty.NilVal, RawState:cty.NilVal, RawPlan:cty.NilVal, Meta:map[string]interface {}(nil)}"
```

### How could Upjet help solve your problem?

Convert the instanceDiff Struct to a JSON object for better debug ability.