feat(rt): allow customizing any async runtime adapter

[0x676e67]

close: #1115

[gemini-code-assist]

Code Review

This pull request refactors the runtime glue and connection management of the client by introducing a unified RuntimeHandle that consolidates task execution, timers, DNS resolution, and connection capabilities, while removing custom connector implementations in favor of wreq-rt traits. The code review highlights three main areas for improvement: first, the conditional compilation of builder and new should be refactored to always-available builder_with_runtime and with_runtime methods to prevent them from being compiled out due to Cargo's additive features; second, the temporary git branches in the [patch.crates-io] section of Cargo.toml must be removed before merging to ensure crates.io publishing is not broken; and third, a doc comment in src/dns/resolve.rs should be updated to correctly state that into_inner returns an owned Box<str> instead of a string slice.