Add support for custom fields in service resource

[sebastianreloaded]

Adds support for custom fields on ipam service ressource.

I added a test for it, but i'm not sure it is enough:

make testacc
...
=== RUN   TestAccNetboxService_customFields
--- PASS: TestAccNetboxService_customFields (2.68s)
...
PASS
coverage: 73.6% of statements
ok  	command-line-arguments	214.713s	coverage: 73.6% of statements

[fbreckle]

We will probably overhaul custom fields in the near future (see the discussion #443 ), but since this implements the current standard, I do not see why I cannot merge this.