feat(buildLog): adding build log hook

[gal-yardeni]

As a part of our effort to open source the CI view in Spinnaker, we have 2 endpoints which power this view:

1. get all builds, based on a project and repo names.
2. for each build, get the build log.

We already added the first hook (#1). This PR is adding the second hook to fetch the build log.
In order to use these hooks, you'll have to implement a CiBuildService in your igor fork, with concrete implementation to those two endpoints.

The CI view will call those endpoints from deck (the UI) via gate.

[gal-yardeni]

cc: @emccallum