Miscellaneous implementation issues with Session handling

[AnthonyGrondin]

I've been using this library for a while and noticed the following miscellaneous issues.
@ivmarkov I'm pinging you since these are related to parts of the library that you contributed.

No handling of TlsError::NoClientCertificate for async Session

TlsError::NoClientCertificate isn't handled for async Session. Instead, the related error code (-29824) is returned when a client makes a request to an async server without providing the required certificates.

Reproduce:

1. Run async_server example with mTLS: SSID=<SSID> PASSWORD=<PASS> cargo +esp esp32s3 --example async_server -Fexamples-async,mtls
2. Make a request to the IP address without providing client certificates
3. Server will fail with:

WARN - MbedTLS error: -29824 / ffff8b80


====================== PANIC ======================
panicked at examples/async_server.rs:240:17:
MbedTlsError(-29824)

---

"Bad usage of mbedtls_ssl_set_bio()" when dropping session

When a client session is dropped, the following warnings occurs:

WARN - 1 (esp-mbedtls/esp-mbedtls-sys/mbedtls/library/ssl_msg.c:2127) Bad usage of mbedtls_ssl_set_bio()
WARN - 1 (esp-mbedtls/esp-mbedtls-sys/mbedtls/library/ssl_msg.c:2847) mbedtls_ssl_flush_output() returned -28928 (-0x7100)
WARN - 1 (esp-mbedtls/esp-mbedtls-sys/mbedtls/library/ssl_msg.c:4877) mbedtls_ssl_write_record() returned -28928 (-0x7100)
WARN - 1 (esp-mbedtls/esp-mbedtls-sys/mbedtls/library/ssl_msg.c:5745) mbedtls_ssl_send_alert_message() returned -28928 (-0x7100)

Reproduce:

1. Build async_client with the following patch:

diff --git a/examples/async_client.rs b/examples/async_client.rs
index 6b3a583..7c003cb 100644
--- a/examples/async_client.rs
+++ b/examples/async_client.rs
@@ -163,7 +163,7 @@ async fn main(spawner: Spawner) -> ! {
         .unwrap()
         .with_hardware_rsa(peripherals.RSA);
 
-    tls.set_debug(0);
+    tls.set_debug(1);
 
     let mut session = Session::new(
         &mut socket,
@@ -207,6 +207,8 @@ async fn main(spawner: Spawner) -> ! {
     println!();
     println!("Done");
 
+    drop(session);
+
     #[allow(clippy::empty_loop)]
     loop {}
 }

2. Run example: SSID=<SSID> PASSWORD=<PASS> cargo +esp esp32s3 --example async_client -Fexamples-async
3. Wait for request and see the above warnings logged in the terminal