AMX Adapter: support native format

[nickjacob]

This PR adds support for native ads & removes unnecessary legacy code that was used for interpolating pixels into ad markup:

• add support for native requests, checking bid type based on bid.ext
• update AMX adapter internal version to 1.2
• add test of bid.ext / bid type logic in MakeBids

[VeronikaSolovei9]

These changes looks good to me. However while I was reviewing and testing this PR I noticed you didn't specify required parameters for imp.ext.amx. In amx.json file there are 2 fields imp.ext.amx can have: tagId and adUnitId. This corresponds to ExtImpAMX.
Without specifying required fields I can run request with empty imp.ext.amx and it doesn't throw error, which I think is not a valid bidder config. I may be wrong, I don't know how your system works inside. If empty imp.ext.amx is a valid case, please ignore this comment. If not - specify required fields. Please refer to PBS documentation and examples: https://docs.prebid.org/prebid-server/developers/add-new-bidder-go.html#bidder-parameters

[nickjacob]

Thanks Veronika!

We do support empty ext.amx. We allow passing the tagId parameter via a custom endpoint that we provide publishers, and adUnitId is not required--that's why we don't require or test for imp.ext.amx.

[mansinahar]

The order for this needs to be the other way round like:

assert.Equal(t, test.bidType, bids.Bids[0].BidType)

The assert function signature requires the expected value to be specified first.
Reference: https://pkg.go.dev/github.com/stretchr/testify/assert#Equal

[mansinahar]

LGTM except for the above minor comment. Also, please update the docs here: https://github.com/prebid/prebid.github.io/blob/master/dev-docs/bidders/amx.md to include native as a supported media type

[nickjacob]

Thanks @mansinahar -- I made the change to the test & opened this PR: prebid/prebid.github.io#3714

[VeronikaSolovei9]

Thank you for your answers. LGTM

[mansinahar]

Thanks @mansinahar -- I made the change to the test & opened this PR: prebid/prebid.github.io#3714

Thanks @nickjacob. Looks good to me