Enable tls credential shell (#8)

Adds about 6kB of code space 😮‍💨 but it's somewhat useful,
though the esp32s3 shows some errors:

```bash
uart:~$ cred list
1001,CA,ERROR,-134
1002,CA,ERROR,-134
1003,CA,ERROR,-134
```

prj.conf

@@ -65,6 +65,11 @@ CONFIG_MBEDTLS_MEMORY_DEBUG=y
 CONFIG_MBEDTLS_SHELL=y
 CONFIG_MBEDTLS_AES_ROM_TABLES=y
 
+# Credential shell, useful for debug
+CONFIG_TLS_CREDENTIALS_SHELL=y
+# Required for TLS crediential shell
+CONFIG_BASE64=y
+
 # More verbose Memfault component logs
 CONFIG_MEMFAULT_LOG_LEVEL_DBG=y