preStart lifecycle event support

[upLukeWeston]

User Story

As a stack creator, I want to be able to define commands that will be run before any containers have been initialized during odo push (preStart events), in the form of Init Containers.

Acceptance Criteria

• It should follow the specification as defined in devfile/kubernetes-api#32 to support execution of the commands based on the events.
• preStart events should run before any of the other containers are started. Refer to design and discussions here for further details.

Links

• Related issue: devfile/kubernetes-api#32
• Proposal for lifecycle events

/kind user-story
/area devfile

[maysunfaisal]

will check if we need this issue or it can be covered by parent issue #3569 in a single sprint

[maysunfaisal]

Had discussion about this on Jul 21'st odo contributors meeting https://docs.google.com/document/d/127JAIs5NJze9W5yVOpULgnD-2HXuGZu1x6_LXGd8keY

To summarize,

Q. Acc to odo proposal doc, preStart translates to init containers. But init containers are mutually exclusive to app containers in a pod acc to Kube doc. So is there a point to preStart if the preStart commands won't have a pvc? Do we rely on devfile authors to be aware of this?

A. Start as init containers, check if access to /projects break sync. We could document it as a warning. Most probably this use case will be  for something external

[maysunfaisal]

Ran into a few issues during implementation of preStart. Raised a few points for discussion here devfile/api#105