Consolidation of changes related to session duration

[jlurien]

What type of PR is this?

• enhancement/feature

What this PR does / why we need it:

Clarification and consolidation of changes affecting management of session duration.

• For creation, duration is made required. Maximum and default are deprecated, relying on the QoS Profile API for any limit. Implementations can grant the requested duration or set a different value in the response.
• maxDuration in QoS Profiles is assumed to be the absolute maximum duration including any extensions. That is, extensions can extend the current session duration to the maximumDuration but no longer.

Session info changes:

• dates are formatted as string, date-format.
• startedAt and expiredAt are both optional and not expected to be returned when qosStatus is "REQUESTED".
• duration is the overall session duration, including any extension. It should be the interval between startedAt and expiresAt, so it is redundant, unless the qosStatus is "REQUESTED". In this case it would reflect the requested or granted duration. For sessions with qosStatus = "UNAVAILABLE", it must be adjusted to the effective duration.

Which issue(s) this PR fixes:

Fixes #291 #249 #266 #280 #281 #288

Special notes for reviewers:

Discussion in #291

Changelog input

- Clarification of concepts and properties related to the management of session duration and session extension

docs

[daniel-dierich]

From a developer point of view this looks good. An internal PoC was successfully created.

[hdamker]

@eric-murray @RandyLevensalor Last meeting we discussed to merge if there are no objections. Do you have checked as well?