fix: unpack the interface type when marshaling response schemas

[childish-sambino]

Fixes this broken test:

=== RUN   TestAccFlexSetup_basic
    resources_flex_test.go:23: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.

        (map[string]string) {
        }

        (map[string]string) (len=1) {
         (string) (len=20) "integration_flow_sid": (string) (len=34) "FW9bda0c9a02677aeabcc7c770b3469b83"
        }
--- FAIL: TestAccFlexSetup_basic (5.05s)

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication