Sovrn: pass-through imp.ext unaltered

[EmilNadimanov]

🔧 Type of changes

• new bid adapter
• bid adapter update
• new feature
• new analytics adapter
• new module
• module update
• bugfix
• documentation
• configuration
• dependency update
• tech debt (test coverage, refactorings, etc.)

✨ What's the context?

#4141 : porting prebid/prebid-server#4413

🧠 Rationale behind the change

Why did you choose to make these changes? Were there any trade-offs you had to consider?

🧪 Test plan

Updated an existing unit test and the integration-test fixtures

🏎 Quality check

• Are your changes following our code style guidelines?
• Are there any breaking changes in your code?
• Does your test coverage exceed 90%?
• Are there any erroneous console logs, debuggers or leftover code in your changes?

[EmilNadimanov]

For context: the Sovrn adapter in prebid-server-go seems to have been overwriting the imp.ext, effectively removing imp.ext.gpid from the payload. This was not the case in the prebid-server-java implementation where gpid was correctly passed-through.
The accepted solution in the original PR does a bit more than just fixing the problem, so I brought the change over to keep code in sync.