add support for GCP PubSub

Largely this is a port of code written for `svix-bridge` in the
`generic-queue` crate.

Adds tests modeled after the rest of the backends here, but with
GCP-specific setup and configuration.

Scheduled delivery is still TODO.

omniqueue/Cargo.toml

@@ -16,13 +16,18 @@ aws-sdk-sqs = { version = "0.25", optional = true }
 bb8 = { version = "0.8", optional = true }
 bb8-redis = { version = "0.13", optional = true }
 futures = { version = "0.3", default-features = false, features = ["async-await", "std"] }
+futures-util = { version = "0.3.28", optional = true }
+google-cloud-default = { version = "0.4.0", features = ["pubsub"], optional = true }
+google-cloud-googleapis = { version = "0.10.0", optional = true }
+google-cloud-pubsub = { version = "0.18.0", optional = true }
 lapin = { version = "2", optional = true }
 rdkafka = { version = "0.29", features = ["cmake-build", "ssl", "tracing"] }
 redis = { version = "0.23", features = ["tokio-comp", "tokio-native-tls-comp", "streams"], optional = true }
 serde = { version = "1", features = ["derive", "rc"] }
 serde_json = "1"
 thiserror = "1"
 tokio = { version = "1", features = ["full"] }
+tokio-util = { version = "0.7", optional = true }
 tracing = "0.1"
 
 [dev-dependencies]
@@ -31,7 +36,13 @@ tokio-executor-trait = "2.1"
 tokio-reactor-trait = "1.1"
 
 [features]
-default = ["memory_queue", "rabbitmq", "redis", "redis_cluster", "sqs"]
+default = ["memory_queue", "gcp_pubsub", "rabbitmq", "redis", "redis_cluster", "sqs"]
+gcp_pubsub = [
+    "dep:futures-util",
+    "dep:google-cloud-googleapis",
+    "dep:google-cloud-pubsub",
+    "dep:tokio-util",
+]
 memory_queue = []
 rabbitmq = ["dep:lapin"]
 redis = ["dep:bb8", "dep:bb8-redis", "dep:redis"]