style(lib): remove default_trait_access lint

[nakaryo716]

Overview

This PR removes default_trait_access = "allow" from Cargo.toml and updates the code to explicitly call type-specific default methods (e.g., ParserConfig::default()) instead of Default::default().

Changes

• Removed default_trait_access = "allow" from Cargo.toml.
• Replaced Default::default() with explicit type-default calls in code.
• Refactored the httparse import in src/server/conn/http1.rs to align with the client-side style.

Questions

Full path for h2 config

In src/client/conn/http2.rs and src/server/conn/http2.rs, I used fully qualified paths to avoid Default::default() while preventing name conflicts:

h2_builder: proto::h2::client::Config::default(),

Question:
Does this look good to you, or would you prefer introducing type aliases (e.g., use proto::h2::client::Config as H2ClientConfig) at the top of the file to keep it shorter?

Import refactoring in src/server/conn/http1.rs

I changed use httparse; to use httparse::ParserConfig;

If you prefer keeping the original import style to minimize the diff, please let me know. I will revert this specific part.

Untouched test codes

I noticed that there are still many Default::default() occurrences in the test codes (e.g., src/proto/h1/role.rs). Currently, the CI clippy job does not check test targets because it runs without --all-targets:

cargo clippy --features full -- -D warnings

As a result, the CI passes with this PR.

Question:
Should I also update the test codes, or should we keep this PR focused only on the library code?

Related issue

Part of a #4071

Sorry for the long description!
If there are any changes needed, please let me know. Otherwise, feel free to merge this PR if everything looks good to you.🙏