fix: do not require default features for axum

[siketyan]

Previously, the connectrpc crate required default features of the axum crate. This created a dependency chain of axum/tokio -> tokio/net -> mio. Since mio does not support WebAssembly (WASM), it was impossible to use this crate in environments like Cloudflare Workers.

Because connectrpc does not actually depend on any of the default features, this PR disables them. This change removes the incompatible dependencies and enables the crate to be used in WASM environments.

No changes are required to the current crate users.

[github-actions]

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[siketyan]

I have read the CLA Document and I hereby sign the CLA