Add invocation ID to root level “invoke” operation span (#3671)

## Motivation and Context
The `invoke` debug span now includes invocation IDs for operation
invocations. It is a random unique ID per client and shared across
retries for the same operation.
 
Example debug trace output with the changes in this PR:
```
2024-05-29T17:50:22.230526Z DEBUG invoke{service=s3 operation=ListObjectsV2 sdk_invocation_id=4652364}:try_op:try_attempt: aws_smithy_runtime_api::client::interceptors::context: entering 'before deserialization' phase
```

## Description
Note that this invocation ID is currently for internal use only to
improve debuggability when looking at logs (usually with customers).
Note also that the invocation ID is not related to [that used in
amz-sdk-invocation-id](https://github.com/smithy-lang/smithy-rs/pull/2626)
(we [attempted to make it
related](https://github.com/smithy-lang/smithy-rs/pull/3661) but decided
to go for this two-way door approach).

In sum, this PR is meant to be simple, and if we are to augment the
functionality (e.g. make the ID configurable or make it related to that
in `amz-sdk-invocation-id`), we can do so without breaking anything.

## Testing
Relies on the existing tests in CI

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