Add 7xbid adapter to compatible with prebid 3.0

[7XBID00]

Type of change

• Bugfix
• Feature
• New bidder adapter
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Does this change affect user-facing APIs or examples documented on http://prebid.org?
• Other

Description of change

I add the 7xbid adapter (it is removed from 3.0 release) and update the adapter to compatible with new version of prebid

• test parameters for validating bids

[
    {
      code: 'test',
      mediaTypes: {
        banner: {
          sizes: [[300, 250], [300,600]],
        }
      },
      bids: [
        {
          bidder: '7xbid',
          params: {
            placementId: 1425292,
            currency: 'USD'

          }
        }
      ]
    },
    {
      code: 'test',
      mediaTypes: {
        native: {
          title: {
            required: true,
            len: 80
          },
          image: {
            required: true,
            sizes: [150, 50]
          },
          sponsoredBy: {
            required: true
          }
        }
      },
      bids: [
        {
          bidder: '7xbid',
          params: {
            placementId: 1429695,
            currency: 'USD'
          }
        },
      ],
    }
  ]

Be sure to test the integration with your adserver using the Hello World sample page.

• contact email of the adapter’s maintainer
• official adapter submission

For any changes that affect user-facing APIs or example code documented on http://prebid.org, please provide:

• A link to a PR on the docs repo at https://github.com/prebid/prebid.github.io/

Other information

[7XBID00]

@robertrmartinez Hi, could you plz process the review

[robertrmartinez]

LGTM!