Put avro dependenciies behind the avro feature flag

Turning avro off drops about 50 crates from the default build, so useful
for development, but the code would need to be cleaned up to remove this
from the default features list

See #163

Cargo.toml

@@ -7,7 +7,6 @@ edition = "2018"
 [dependencies]
 anyhow = "1"
 async-trait = "0.1"
-apache-avro = "^0.14"
 base64 = "0.13"
 bytes = "1"
 chrono = "0.4.31"
@@ -20,7 +19,6 @@ lazy_static = "1"
 log = "0"
 maplit = "1"
 rdkafka = { version = "0.28", features = ["ssl-vendored"] }
-schema_registry_converter = { version = "3.1.0", features = ["easy", "json", "avro"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 strum = "0.20"
@@ -46,13 +44,21 @@ rusoto_s3 = { version = "0.47", default-features = false, features = ["rustls"],
 # sentry
 sentry = { version = "0.23.0", optional = true }
 
+# avro
+apache-avro = { version =  "^0.14", optional = true }
+schema_registry_converter = { version = "3.1.0", optional = true, features = ["easy", "json", "avro"] }
+
 # azure feature enabled, mostly used for tests
 azure_core = { version = "0.18.0", optional = true }
 azure_storage = { version = "0.18.0", optional = true }
 azure_storage_blobs = { version = "0.18.0", optional = true }
 
 [features]
-default = []
+default = ["avro"]
+avro = [
+    "apache-avro",
+    "schema_registry_converter",
+]
 sentry-ext = ["sentry"]
 dynamic-linking = [ "rdkafka/dynamic-linking" ]
 azure = [