Update custom User-Agent header to include the version

[jameswiltshire]

The user-agent header added by the package looks like "User-Agent":"@gradientedge/commercetools-utils" - it would be useful to include the version/release number alongside this, and it should follow the syntax of User-Agent: <product> / <product-version> <comment>
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent

Suggested format here could be User-Agent: gradientedge-commercetools-utils/1.2.3
(and can include any optional provided systemIdentifier as a suffix to this)

[jimmythomson]

The package version is the tricky bit. When the code is packaged for publishing, we do not know what the package version number will be, and so cannot add it in to the codebase. We also cannot rely on being able to read the package.json file at run time because we may be in a browser environment, or, the code may have been built and bundled, losing access to the package.json file.

I'll have a think about other ways of doing this. It almost definitely has to be built in to the code that's published to npm. We can use webpack to replace an env var with a version number, but as I say, we don't have that version number anywhere in the code or the package.json file.

[jimmythomson]

Looks like this semantic-release plugin might do the trick: https://github.com/google/semantic-release-replace-plugin