Add SAFE certificate extension

[lex-node]

Motivation

• The codebase and conversion logic expect a SAFE certificate extension under src/storage/extensions to provide SAFE-specific metadata, but no extension file existed, causing the referenced PR flow to fail.

Description

• Added src/storage/extensions/SAFEExtension.sol which defines a SAFEData struct and a SAFEExtension contract implementing ICertificateExtension with decodeExtensionData, encodeExtensionData, supportsExtensionType, and getExtensionURI that emits SAFE metadata under the SAFEDetails JSON key.
• The contract is UUPS-upgradeable and integrates BorgAuthACL with initialize and _authorizeUpgrade restricted to onlyOwner and includes a uint256ToString helper for JSON encoding.

Testing

• No automated tests were executed for this change.

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Codex Task