Add randomizedTriggerRate to confidence field

[mwjacksonmsft]

Including a randomizedTriggerRate in the proposal per feedback from w3c/navigation-timing#202 (comment)

Including this provides a few benefits:

• Different browsers can choose more or less strict values
• The value can drift over time if we think our initial choices were wrong
• Ensures consumers can interpret the data properly (which is important for the debiasing step).