SQLx 0.9 upgrade requires runtime/TLS feature migration and dependency cleanup

[nivek-ph]

Problem

PR #108 updates the sqlx requirement from 0.8 to 0.9, but SQLx 0.9 removed the combined runtime/TLS feature names currently used by this crate.

Changing only:

sqlx = { version = "0.9", default-features = false, features = ["macros"] }

fails dependency resolution with errors such as:

package `sqlx-adapter` depends on `sqlx` with feature
`runtime-async-std-native-tls` but `sqlx` does not have that feature

Proposed changes

Keep the existing public feature names for downstream compatibility, but map them to SQLx 0.9 features:

runtime-async-std-native-tls = [
    "casbin/runtime-async-std",
    "sqlx/runtime-async-std",
    "sqlx/tls-native-tls",
]

runtime-async-std-rustls = [
    "casbin/runtime-async-std",
    "sqlx/runtime-async-std",
    "sqlx/tls-rustls",
]

runtime-tokio = [
    "casbin/runtime-tokio",
    "sqlx/runtime-tokio",
]

runtime-tokio-native-tls = [
    "casbin/runtime-tokio",
    "sqlx/runtime-tokio",
    "sqlx/tls-native-tls",
]

runtime-tokio-rustls = [
    "casbin/runtime-tokio",
    "sqlx/runtime-tokio",
    "sqlx/tls-rustls",
]

The upgrade is also an opportunity to remove unused direct dependencies:

• The optional tokio and async-std dependencies are not enabled by the crate's runtime features, and production code does not use them directly. The dev-dependencies are sufficient for the test macros.
• dotenvy is only used by SqlxAdapter::new() to call dotenv().ok(). The adapter already receives a complete database URL, so loading the caller's .env is an unnecessary hidden side effect.

The plain runtime-tokio feature should also attach the existing Tokio tests. Currently, only the TLS variants receive #[tokio::test].

Acceptance criteria

• PostgreSQL builds with Tokio and async-std.
• Native TLS and rustls variants compile.
• runtime-tokio without a TLS backend runs the existing tests.
• SqlxAdapter::new_with_pool(pool.clone()) accepts a SQLx 0.9 pool.
• Policies saved through a shared pool can be loaded by another adapter/Enforcer using the same pool.
• Existing policy add/remove tests continue to pass.
• SQLx 0.9 offline metadata is generated for PostgreSQL, MySQL, and SQLite.
• A downstream application using SQLx 0.9 resolves one SQLx version rather than both 0.8 and 0.9.

Related: #108, #109.