[mbedtls3] `mbedlts` would breaks the record size limit when sending big record

[Taowyoo]

About TLS record limit, current upstream mbedtls has some problems that cause bugs:

1. In TLS 1.3, common TLS implementations now send record_size_limit extension to negotiation max record size between client and server. But:mbedtls currently (ver: 3.4.0) does not support it and just ignore that extension.
2. When need to send big record, mbedlts will send a TLS record with record length > 2^14 which breaks the RFC, as a result, causing client's record overflow error.

It has been ensure problem 2 cause: clients (especially common browsers) will throw "record_overflow" alert to server when server try to send a record that bigger than 2^14 .

From wireshark, I saw it sends a record with length field value: 16416 which is 32 bigger the 2^14.

So current temporary solution is to manually set the MBEDTLS_SSL_OUT_CONTENT_LEN to a smaller number, see #294

This may also related to the our async_write function under mbedtls/src/ssl/context.rs.

But further investigation is needed to identify real reason for this.
Ref:
https://www.rfc-editor.org/rfc/rfc8446#section-5.1
https://datatracker.ietf.org/doc/html/rfc8449

[jethrogb]

#294 is just a workaround. Bug remains.

[jethrogb]

Upstream report Mbed-TLS/mbedtls#7918