Set `User-Agent` header

[gr2m]

Setting a custom user agent is considered best practice and would help debug problems in case any occur. As far as I can tell, ai is not setting a custom user agent right now.

The user agent should include the most important versions of ai and @ai-sdk/* packages, and user should be able to add their own custom header to it.

Octokit's implementation worked out well over the years:
https://github.com/octokit/core.js/blob/9c425e32f001330786925c0a1d3c47dbcb20103f/test/request.test.ts#L53-L73

[gr2m]

The user agent should include the most important versions of ai and @ai-sdk/* packages, and user should be able to add their own custom header to it.

When installing ai, we currently also install

• @ai-sdk/provider-utils
• @ai-sdk/provider
• @ai-sdk/gateway
• @opentelemetry/api
• zod

I think for the scope of this issue, it will be enough to just pass the ai version.

In the future, I would also add the version of @ai-sdk/provider-utils. And for @ai-sdk/gateway we should set it only when the gateway is used. Ideally we would also set the version the provider package that is used in a request.