feat: export Connection type and document with_pool() for sharing an existing db pool

[Copilot]

Users had no clear way to share an existing r2d2::Pool with DieselAdapter. The with_pool() constructor already existed but was undiscoverable — the Connection type alias it requires was internal, and there was no documentation or example showing its use.

Changes

• src/lib.rs — Re-export actions::Connection so users can correctly type their pool as Pool<ConnectionManager<Connection>> without knowing the underlying concrete connection type
• src/adapter.rs — Add doc comments to new() and with_pool(); with_pool() includes a runnable example; add test_with_pool unit test
• README.md — Split the Example section into two subsections covering both constructors

Usage

use diesel::r2d2::{ConnectionManager, Pool};
use diesel_adapter::{casbin::prelude::*, Connection, DieselAdapter};

let database_url = std::env::var("DATABASE_URL").expect("DATABASE_URL must be set");
let manager = ConnectionManager::<Connection>::new(database_url);
let pool = Pool::builder().max_size(8).build(manager).unwrap();

// Pass your application's existing pool directly
let a = DieselAdapter::with_pool(pool)?;
let mut e = Enforcer::new(m, a).await?;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use my existing Db pool for constructing diesel_adapter::DieselAdapter</issue_title>
<issue_description>I want to share my app's db pool to casbin, is it possible to have a new constructor that takes diesel:r2d2:Pool as parameter instead of taking number of connections?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Use my existing Db pool for constructing diesel_adapter::DieselAdapter #97

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