Adding gen_ai.tool.description as additional attribute for agent's tool calling 

### Area(s)

area:gen-ai

### What's missing?

**Area(s)**

area:gen-ai

**What's the problem?**

A follow up for https://github.com/open-telemetry/semantic-conventions/pull/2059/files

**Describe the solution you'd like**

Adding gen_ai.tool.description for invocation of any tool calling by agents

### Describe the solution you'd like

_No response_