Update: Removed browser functionality (see below for reasons)

As a follow-up of an incident (quite a while ago, but this unfortunately slipped through), SDKs were asked to send a user-agent Http header to determine by just looking at incoming request headers which SDK the request is coming from. See develop specification.

This PR makes the following changes to sending user agent HTTP headers with envelope requests made by the transport:

• Send user-agent in all server-runtime SDKs
• Extract the headers option from individual transport options to BaseTransportOptions. This allows us to type-safely add the user agent header to the SDKs transport options which is the easiest way to pass a header to the transport without having to modify or extend any of the existing transport APIs. I checked and every transport implementation we currently export exposed a headers option anyway, so this just unifies it. Given this is an optional property, custom transport implementations extending BaseTransportOptions won't break either. The problem here is only that they might not actually support this option which all things considered I think is fine. If reviewers have different opinions, I'm happy to revisit this.
• Unit and integration/e2e tests for node and cloudflare

This PR makes no change to the sdk envelope header which carries the same (more detailed) information (but in the request body)

Why not in Browser SDKs?

1. Relay infers the user's browser information based on user-agent, so we cannot set a custom user agent name that overwrites it.
2. We also cannot add a custom header (something like x-sentry-user-agent) because then the SDK would send a non-standard header which could block all requests to Sentry, depending on the CORS setup of the application the SDK is added to. This is especially relvant for users who use tunnel and forward the SDK requests to a proxy server.

ref: https://linear.app/getsentry/project/chore-alignextend-user-agents-795bfddfa217/overview