Add StartTransaction Function

[smeubank]

Summary

Today the Sentry Go SDK only provides support for StartSpan. However according to the Sentry model span(s) are sent to Sentry in transactions.

StartTransaction function should be added to Go according to our develop documentation for SDKs.

Guidelines for Performance Monitoring

Sentry.startTransaction function requirements

Consider proposed changes currently being proposed here, regarding new manually created transactions automatically being attached to an existing trace or not

Motivation

To improve the developer experience when instrument you application for Performance monitoring we want to provide functions so that developer do not need to manual construct HTTP requests.

Additional Context

This is a crucial piece on the road to getting the GO SDK to have more feature parity for the Performance similar to other SDKs.