Reduce dependency (axum)

[hirofumi]

Would it be possible to enable tonic's channel feature instead of transport feature? transport feature depends on axum (via server feature and router feature) and it seems to be unnecessary for gcp-bigquery-client.

The features of tonic are as follows.

[features]
codegen = ["dep:async-trait"]
gzip = ["dep:flate2"]
zstd = ["dep:zstd"]
default = ["transport", "codegen", "prost"]
prost = ["dep:prost"]
tls = ["dep:rustls-pemfile", "dep:tokio-rustls", "dep:tokio", "tokio?/rt", "tokio?/macros"]
tls-roots = ["tls-native-roots"] # Deprecated. Please use `tls-native-roots` instead.
tls-native-roots = ["tls", "channel", "dep:rustls-native-certs"]
tls-webpki-roots = ["tls", "channel", "dep:webpki-roots"]
router = ["dep:axum", "dep:tower", "tower?/util"]
server = [
  "router",
  "dep:async-stream",
  "dep:h2",
  "dep:hyper", "hyper?/server",
  "dep:hyper-util", "hyper-util?/service", "hyper-util?/server-auto",
  "dep:socket2",
  "dep:tokio", "tokio?/macros", "tokio?/net", "tokio?/time",
  "tokio-stream/net",
  "dep:tower", "tower?/util", "tower?/limit",
]
channel = [
  "dep:hyper", "hyper?/client",
  "dep:hyper-util", "hyper-util?/client-legacy",
  "dep:tower", "tower?/balance", "tower?/buffer", "tower?/discover", "tower?/limit", "tower?/util",
  "dep:tokio", "tokio?/time",
  "dep:hyper-timeout",
]
transport = ["server", "channel"]

https://github.com/hyperium/tonic/blob/v0.12.3/tonic/Cargo.toml#L25-L54

[lquerel]

I will include this PR in the next release. Thanks