SYN-5402: client certificates

[borisvalo]

Resolves #SYN-5402

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Before the change?

• syntheticsclient/v2 does not expose Synthetics client certificate CRUD APIs.
• HTTP V2, Browser V2, and API V2 client models do not expose client certificate attachment fields.
• Public request details can include raw API tokens, certificate content, passwords, header values, cookie values, or authentication values.

After the change?

• Adds public ClientCertificate* models using public API JSON casing.
• Adds create, get, list, update, and delete methods for /certificates.
• Adds HTTP certificateId, Browser advancedSettings.certificateIds, and API request configuration.certificateId model support.
• Redacts API token, content, password, and value fields from captured request details.
• Removes raw request dump printing from the public API client path.

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: 71.8% of statements
ok github.com/splunk/syntheticsclient/v2/syntheticsclientv2

Does this introduce a breaking change?

• Yes
• No

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