[Feature]: add sdk version to user agent string

[yaythomas]

What would you like?

1. User Agent Format
   The SDK MUST append a custom user agent entry in the following format:

{language-sdk-package-name}/{version}

Where:

{language-sdk-package-name} is the SDK identifier (ie: @aws/durable-execution-sdk-js)
{version} is the semantic version of the SDK (e.g., 1.0.2)
2. Version Source
The version MUST be:

Embedded at build time or runtime
Kept in sync with the published SDK version

See aws/aws-durable-execution-sdk-js#432

As implemented:
aws/aws-durable-execution-sdk-js#440

Possible Implementation

No response

Is this a breaking change?

No

Does this require an RFC?

No

Additional Context

No response

[Iamrodos]

My PR may be relevant #293

It exports version from the package root, so the version is accessible via the standard Python convention (aws_durable_execution_sdk_python.version). Currently it requires importing from the internal about submodule.

Not strictly a blocker for this issue. Whilst the user agent implementation could import from about directly, having the version exposed at the package level would make it cleaner and aligns with how boto3/botocore handle it.

Be great to see #293 merged. 😄

[yaythomas]

aws/aws-durable-execution-sdk-java#160