switch agent field to be a map instead of only queue

[com6056]

Buildkite agent tags can be arbitrary key/value pairs, so let's switch that field to support that 🎉

make test
go test -race -coverprofile=coverage.out -covermode=atomic
PASS
coverage: 86.7% of statements
ok  	github.com/chronotc/monorepo-diff-buildkite-plugin	0.258s
docker-compose run --rm plugin_test
Creating monorepo-diff-buildkite-plugin_plugin_test_run ... done
 ✓ Notify when the plugin cannot be parsed
 ✓ Pipeline is generated without wait property
 ✓ Pipeline is generated with build config from env
 ✓ Pipeline is generated with all options

4 tests, 0 failures

make quality
go vet
go fmt
go mod tidy
docker-compose run --rm plugin_lint
Creating monorepo-diff-buildkite-plugin_plugin_lint_run ... done
TAP version 13
ok 1 - plugin.yml is valid
ok 2 - All the readme config examples for plugin id 'chronotc/monorepo-diff' are valid (4 found)
ok 3 - Readme version numbers are up-to-date (v2.2.0)
1..3
# time=82.974ms