Add example and documentation for custom tool

**Is your feature request related to a problem? Please describe.**
The [langchainjs docs](https://js.langchain.com/docs/how_to/custom_tools/) do a good job of showing how to turn a function (just a regular JS function) into a tool. The example is simple enough that it is easy to understand how to create your own tool.

**Describe the solution you'd like**
Create an the example for the adder tool but in langchain_rust. Add it to examples and README.md

```javascript
import { z } from "zod";
import { tool } from "@langchain/core/tools";

const adderSchema = z.object({
a: z.number(),
b: z.number(),
});
const adderTool = tool(
async (input): Promise => {
const sum = input.a + input.b;
return `The sum of ${input.a} and ${input.b} is ${sum}`;
},
{
name: "adder",
description: "Adds two numbers together",
schema: adderSchema,
}
);

await adderTool.invoke({ a: 1, b: 2 });
```

**Describe alternatives you've considered**

**Additional context**
The example should focus on the langchain-rust boilerplate necessary for any tool with any explanations which are specific to rust version of langchain.

[Example code](https://github.com/dfberry/rust/pull/18)

I can submit a PR.

Look at the existing examples directory in the repository to understand the structure. The task is to create a Rust equivalent of the provided JavaScript adder tool example. Start by examining how other tools are defined in langchain-rust, likely in a tools module. The example should be placed in the examples folder and referenced in README.md. Test the example by running it with cargo run --example adder_tool (or similar) to ensure it works.

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