Enable HTTP connection reuse for okhttp

[rfc2822]

Purpose

Enable HTTP connection reuse and keep-alive for WebDAV connections to improve performance and reduce connection overhead.

This only applies to okhttp connections and not to Ktor, which manages engine client reuse itself (can be verified / tested at a later time).

It applies to all HTTP operations, so both sync and WebDAV.

Short description

• Create a shared OkHttpClient instance for connection pooling
• Utilize sharedOkHttpClient.newBuilder() for new client instances instead of creating new clients from scratch
• Move timeout configuration to the shared client to ensure consistent settings
• Remove the now-redundant buildTimeouts() method

This change allows OkHttp to reuse connections and share thread pools across different HTTP clients, which is especially beneficial for WebDAV operations that often involve multiple sequential requests to the same server.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[sunkup]

Did not verify whether connections are now shared, but the test looks good to me and sync still works :)