feat(datadog-aws-lambda): add root invocation span with OTel tracing (#213)

> **PR Stack:** #194 (workspace setup) -> **#213 (lambda root span)** ->
~~#190~~#219 (lambda inferred spans)

# What does this PR do?

Implements the core Lambda handler wrapper for `datadog-aws-lambda`.
Each invocation is automatically instrumented with an `aws.lambda` root
span carrying cold_start, request_id, function metadata, and error
recording.

This PR intentionally does **not** include trigger extraction or
inferred spans. Those are layered on in ~~#190~~#219 with minimal API
surface changes.

## Usage

```rust
lambda_runtime::run(TracedService::new(my_handler, Config::default())).await
```

## What's included

- **`TracedService`** - `tower::Service` that wraps lambda handler with
OTel tracing
- **`LambdaSpan`** - `aws.lambda` root span with `cold_start`,
`request_id`, `function_arn`, `function_version`, `functionname`,
`_dd.origin=lambda`
- **`Invocation`** - start/handler_context/finish lifecycle with error
recording
- **`Config`** - `service`/`env`/`version` or full
`DatadogTracingBuilder` control
- Lambda-appropriate OTel defaults: synchronous writes, no client-side
stats

# Motivation

This PR establishes the root invocation tracing that ~~#190~~#219 builds
inferred spans on top of.


Ref: #221

---------

Co-authored-by: Yury Gribkov <yury.gribkov@gmail.com>

Author: Dogbu-cyber