Function create_signed_platform_claim 

pub fn create_signed_platform_claim(
    platform: &str,
    namespace: &str,
    did: &str,
    key_alias: &str,
    passphrase_provider: &dyn PassphraseProvider,
) -> Result<String>

Creates a signed platform claim linking a DID to a platform username.

The claim is JSON-canonicalized (RFC 8785) before signing, ensuring deterministic verification without the original OAuth token.

Args:

• platform: Platform name (e.g., “github”).
• namespace: Username on the platform.
• did: The controller DID (e.g., “did:keri:E…”).
• key_alias: Keychain alias for the signing key.
• passphrase_provider: Provider for key decryption passphrase.

Usage:

let claim_json = create_signed_platform_claim("github", "octocat", "did:keri:E...", "main", provider)?;