Allow nested statements by overwriting their types with string

[TiberiuGC]

Issue #, if available:

Description of changes:

• on top of, and to be reviewed after Add support for WebACL CRD #7
• nested statements are allowed in yaml files via strings e.g.

      statement:
        andStatement: |
          statements:
            - labelMatchStatement:
                scope: "RULE_SET"
                key: "aws:UserAgent"
                value: "Mozilla"
            - ipSetReferenceStatement:
                arn: "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"

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This pull-request has been approved by: TiberiuGC
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@TiberiuGC: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
wafv2-verify-attribution	b816aec	link	true	/test wafv2-verify-attribution

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