Add ConfidenceLevel property to ContentLinkHandler

[DevGary]

Add ConfidenceLevel to ContentLinkHandler so it can state how confident it is at handling the link. This would allow users of library to filter out handlers based on their desired confidence level.

enum ConfidenceLevel {
    HIGH, // Uses Official APIs
    MEDIUM, // Uses Unofficial APIs
    LOW, // Uses url to parse Content
}

[DevGary]

ConfidenceLevel should probably instead be a param passed to ContentLinkHandler canHandleLink() and getContent() as a ContentLinkHandler can have differing ConfidenceLevels depending on the link. eg StreamableContentLinkHandler may be able to get Content either via the official API or by attempting to load and parse the webpage.