Implements sharing a route across spaces

[klakin-pivotal]

This commit adds 'POST /v3/routes/:guid/relationships/shared_spaces'.
The endpoint's behavior currently matches service instance sharing behavior.
Route sharing does not handle isolation segments.

This endpoint is currently experimental.

This commit also makes minor changes to the error message transmitted
when you fail to share a Service Instance. The existing message didn't
tell you what instance failed to share, or the space that caused the
failure, which are both nice things for an operator to know.

Here is a note about some implementation detail for future programmers:

Route.shared? does not make use of the class's shared_spaces
member variable. This is because shared_spaces is an Array, which
indicates that -behind the scenes- Sequel will first fetch all of the
data for all of the matching objects in the association, wrap that in
an array, and then hand it to us. Given that we only want to know if
there are more than zero items in that array, this could be a huge
waste on Foundations that have a route shared between many spaces.

So, we explicitly search for Spaces associated with the Route and tell
the ORM to tell us if there is more than one. The Sequel docs claim that
using empty? on a Dataset object implicitly adds in a LIMIT 1
to the query that the ORM generates, so the code as written should only
ever send us one row at most.

For reference, this Pivotal Tracker story tracks this work, but it may not be accessible for folks who aren't a VMWare employee: https://www.pivotaltracker.com/story/show/181734583

Co-authored-by: Alex Rocha [email protected]
Co-authored-by: Merric de Launey [email protected]
Co-authored-by: Kenneth Lakin [email protected]

==============================================

Thanks for contributing to cloud_controller_ng. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  See the description above

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: xandroc / name: Alex (8934f06)