Added Confidence Rating to ECS

[secops4thewin]

Added the following line to threat ECS

• threat.confidence
  The confidence rating of the threat, ranging from 0 - 100, following STIX 2.0 confidence rating

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f2fd6f7, b60752f

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[secops4thewin]

I have now signed the contributor agreement :)

[ebeahan]

Hi @secops4thewin 👋

If you want to propose adding the threat.confidence field, feel free to join into the discussions in-progress over in #1127 concerning the threat.* RFC.

[ebeahan]

@shimonmodi @dcode @peasead - this is a proposed field addition to the threat.*. Let's continue the discussion in #1127, but I wanted to call attention to the suggestion before closing.

[peasead]

#1127 (comment)