AMX: Support overriding seat and demandSource

[nickjacob]

PBS equivalent of https://github.com/prebid/Prebid.js/pull/11712

For publishers that set allowAlternateBidderCodes, optionally override .Seat and/or .meta.demandSource. This provides additional transparency/reporting capabilities.

[github-actions[bot]]

Code coverage summary

Note:

• Prebid team doesn't anticipate tests covering code paths that might result in marshal and unmarshal errors

• Coverage summary encompasses all commits leading up to the latest one, 43a77b2c2f6a68300c59bb98a5abe5986a9d1006

  amx

  Refer here for heat map coverage report

  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:25:  Builder         100.0%
  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:49:  ensurePublisherWithID   100.0%
  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:60:  MakeRequests        93.3%
  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:122: MakeBids        96.6%
  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:185: getBidExt       100.0%
  github.com/prebid/prebid-server/v2/adapters/amx/amx.go:195: getMediaTypeForBid  100.0%
  total:                              (statements)        96.4%