fix(flex): add Stringify function and tests

[padamstx]

This PR adds the Stringify() function and its unit tests to the flex module. This new function will be used by Terraform code emitted by the SDK generator.

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Output from acceptance testing:

$ cd ibm/flex && go test -v -run TestStringify
=== RUN   TestStringifyString
--- PASS: TestStringifyString (0.00s)
=== RUN   TestStringifyBoolean
--- PASS: TestStringifyBoolean (0.00s)
=== RUN   TestStringifyNumber
--- PASS: TestStringifyNumber (0.00s)
=== RUN   TestStringifySlice
--- PASS: TestStringifySlice (0.00s)
=== RUN   TestStringifyMap
--- PASS: TestStringifyMap (0.00s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/flex	0.012s
...

[padamstx]

@hkantare Please review and merge when you can. This change is needed by a generator PR which we'll wait and merge after this one. Thanks!