feat: expose defineFormat on Genkit instance, allowing to define custom formats

[pavelgj]

    ai.defineFormat(
      {
        name: 'banana',
        format: 'banana',
      },
      (schema) => {
        let instructions: string | undefined;

        if (schema) {
          instructions = `Output should be in banana format`;
        }

        return {
          parseChunk: (chunk) => {
            return `banana: ${chunk.content[0].text}`;
          },

          parseMessage: (message) => {
            return `banana: ${message.content[0].text}`;
          },

          instructions,
        };
      }
    );