Add pool balancing strategy

This change adds the ability to specify the pool balancing strategy to
use when processing queued jobs. Before this change, GARM would round-robin
through all pools that matched the set of tags requested by queued jobs.

When round-robin (default) is used for an entity (repo, org or enterprise)
and you have 2 pools defined for that entity with a common set of tags that
match 10 jobs (for example), then those jobs would trigger the creation of
a new runner in each of the two pools in turn. Job 1 would go to pool 1,
job 2 would go to pool 2, job 3 to pool 1, job 4 to pool 2 and so on.

When "stack" is used, those same 10 jobs would trigger the creation of a
new runner in the pool with the highest priority, every time.

In both cases, if a pool is full, the next one would be tried automatically.

For the stack case, this would mean that if pool 2 had a priority of 10 and
pool 1 would have a priority of 5, pool 2 would be saturated first, then
pool 1.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

runner/enterprises_test.go

@@ -78,6 +78,7 @@ func (s *EnterpriseTestSuite) SetupTest() {
 			name,
 			fmt.Sprintf("test-creds-%v", i),
 			fmt.Sprintf("test-webhook-secret-%v", i),
+			params.PoolBalancerTypeRoundRobin,
 		)
 		if err != nil {
 			s.FailNow(fmt.Sprintf("failed to create database object (test-enterprise-%v)", i))