Union type over the five A2A security scheme types.

Each scheme maps to a discriminator key in the wire format:

• "apiKeySecurityScheme" → %APIKey{}
• "httpAuthSecurityScheme" → %HTTPAuth{}
• "oauth2SecurityScheme" → %OAuth2{}
• "openIdConnectSecurityScheme" → %OpenIDConnect{}
• "mtlsSecurityScheme" → %MutualTLS{}