New Data Source/Resource: teamcity_agent_pool & teamcity_agent_pool_project_assignment

[tombuildsstuff]

This PR includes a new Data Source & Resource for Agent Pools and a Resource for managing Agent Pool - Project assignments - which can be managed via:

data "teamcity_agent_pool" "test" {
  name = "Default"
}

resource "teamcity_agent_pool" "test" {
  name = "Example"
}

resource "teamcity_agent_pool_project_assignment" "test" {
  agent_pool_id                 = teamcity_agent_pool.test.id
  project_id                    = teamcity_project.test.id
  disassociate_from_other_pools = true
}

Dependent on cvbarros/go-teamcity#79 - but should compile & the tests pass once that's available 😄