Add content impression and interaction tracking

[aeirola]

Should be straightforward to just call the underlying SDKs directly.

trackContentImpression(name: string, piece?: string, target?: string)
trackContentInteraction(name: string, piece?: string, target?: string)

See:

• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_Android.html#tracking-content-impressions-and-interactions
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_iOS.html#tracking-content-impressions-and-interactions