feat(js): added dynamicTool factory function that does not depend on genkit instance

[pavelgj]

import { dynamicTool } from 'genkit';

const dynamicGablorkenTool = dynamicTool(
  {
    name: 'dynamicGablorkenTool',
    inputSchema: z.object({
      value: z.number(),
    }),
    description: 'can be used to calculate gablorken value',
  },
  async (input) => {
    return input.value * 3 - 4;
  }
);

const { text } = ai.generateStream({
  model: googleAI.model('gemini-2.0-flash'),
  tools: [dynamicGablorkenTool],
  prompt: `what is a gablorken of ${input}`,
});

also added global (Genkit instance level) context:

const ai = genkit({
  context: { something: 'extra' }
});

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)