SYN-4606: Add WaitForNavTimeoutDefault and MaxWaitTimeDefault

[Pajk]

Resolves SYN-4606

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Before the change?

• No way how to find out if the WaitForNavTimeout and MaxWaitTime browser check step fields were set to some value or the default value is being used. This information is needed in the Synthetics Terraform provider to properly handle default values.

After the change?

• Added WaitForNavTimeoutDefault and MaxWaitTimeDefault boolean flags. When true, the value in the respective attributes is the default one provided by the API. If false, the value was set to the value by the user in UI or through the Synthetics API.

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

PASS
coverage: 70.3% of statements
ok      github.com/splunk/syntheticsclient/v2/syntheticsclientv2        1.343s  coverage: 70.3% of statements

Does this introduce a breaking change?

• Yes
• No

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