SYN-6665: MFA ToTP client

[borisvalo]

Resolves SYN-6665

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Before the change?

• syntheticsclientv2 did not expose the Synthetics /totps API, so Terraform/provider work could not manage TOTP variables through the released client.
• RequestDetails.RequestBody could retain sensitive request data, including auth headers and secret request body fields.
• RequestDetails.RawRequest retained the original request, which could expose API tokens or request-body secrets.

After the change?

• Adds syntheticsclientv2 TOTP variable models and CRUD methods for /totps.
• Supports create, get, list, update, and delete using the confirmed API roots: totp and totps.
• Redacts sensitive request details, including auth/cookie headers, JSON secret, password, and content fields, browser header values, and browser cookie values.
• Stops retaining RawRequest in RequestDetails to avoid exposing original auth headers or secret request bodies.
• Adds unit coverage for the TOTP API contract, update pointer semantics, redaction behavior, and non-over-redaction of benign values.

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

Client-only change; no live acceptance tests were run. Unit tests:

go test ./syntheticsclientv2 -tags=unit_tests -run 'TestSanitizeRequestDumpStoredInRequestDetails|Test(Create|Update)TotpVariableV2|TestMakePublicAPICallStoresRedactedRequestDetailsForTotpVariableCRUD' -count=1
ok

go test ./syntheticsclientv2 -tags=unit_tests -count=1
ok

Does this introduce a breaking change?

• Yes
• No

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