Add TLS record fragmentation

[Lanius-collaris]

No description provided.

[SadeghHayeri]

Hello,
sorry for late response, what does this PR for? the main functionally of fragmenting TLS record already exist in the code (fragment first packet will be fragmented and this is fundamental of GT)
Can you explain it more?

[Lanius-collaris]

Both TCP and the record layer of TLS support fragmentation, this PR is for the latter.

Ref:
Circumventing the GFW with TLS Record Fragmentation

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Handshake messages MAY be coalesced into a single TLSPlaintext record or fragmented across several records…

RFC 8446 section-5.1

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Data Structures at RFC 8446 Appendix-B

uint16 ProtocolVersion;
enum {
      invalid(0),
      change_cipher_spec(20),
      alert(21),
      handshake(22),
      application_data(23),
      heartbeat(24),  /* [RFC 6520](https://datatracker.ietf.org/doc/html/rfc6520) */
      (255)
} ContentType;

struct {
      ContentType type;
      ProtocolVersion legacy_record_version;
      uint16 length;
      opaque fragment[TLSPlaintext.length];
} TLSPlaintext;