Move lifetime to IG attribute

[JensenPaul]

This was done in the spec several months ago but the explainer was never updated. This change was based on feedback that times in Web APIs should be specified in milliseconds, and moving it to the interest group itself was more extensible.

Note that Chrome continues to accept the lifetime when specified in either place.

[caraitto]

We need to fix this one too:

turtledove/FLEDGE.md

Line 250 in 433120f

Indicating whether the origin in the path has permissions to join and/or leave interest groups owned by the domain the request is sent to. Missing permissions are assumed to be false. Since calling `navigator.joinAdInterestGroup()` with a duration of 0 effectively leaves an interest group, `joinAdInterestGroup: true` also allows an origin to call navigator.leaveAdInterestGroup(), even if `leaveadInterestGroup` is missing or is set to false. Note that both `leaveAdInterestGroup()` and `clearOriginJoinedAdInterestGroups()` check the "leaveAdInterestGroup" permission.

"Since calling navigator.joinAdInterestGroup() with a duration of 0 effectively leaves an interest group"

[JensenPaul]

We need to fix this one too:

turtledove/FLEDGE.md

Line 250 in 433120f

Indicating whether the origin in the path has permissions to join and/or leave interest groups owned by the domain the request is sent to. Missing permissions are assumed to be false. Since calling `navigator.joinAdInterestGroup()` with a duration of 0 effectively leaves an interest group, `joinAdInterestGroup: true` also allows an origin to call navigator.leaveAdInterestGroup(), even if `leaveadInterestGroup` is missing or is set to false. Note that both `leaveAdInterestGroup()` and `clearOriginJoinedAdInterestGroups()` check the "leaveAdInterestGroup" permission.

"Since calling navigator.joinAdInterestGroup() with a duration of 0 effectively leaves an interest group"

Fixed.