feat: add MPTCP support

[AkinoKaede]

Background

Multipath TCP or MPTCP is an extension to the standard TCP and is described in RFC 8684. It allows a device to make use of multiple interfaces at once to send and receive TCP packets over a single MPTCP connection. MPTCP can aggregate the bandwidth of multiple interfaces or prefer the one with the lowest latency.

Support for Multipath TCP was first added in Go 1.21. In a future Go release we may enable Multipath TCP by default on systems that support it.
Go 1.21 Release Note

Other proxy softwares that written in Go had add it.

Behavior

If mptcp is not set in sockopt, V2Ray will not do any action about MPTCP, and MPTCP is disabled by default on latest Go currently. If mptcp is set to true, V2Ray will try to enable MPTCP, but it only works on Linux now. On other platforms, it will make any impact.

If one of the server and the client does not support MPTCP, MPTCP will not work and the connection will be the same as a normal TCP connection.

Go versions

This PR can only be compiled with Go 1.21 or newer versions. The minimal Go version in v2ray-core and quic-go in main branch is Go 1.21, so I think this PR not make any impact to this problem.

v2ray-core/go.mod

Line 3 in 8191faa

go 1.21

Test Results

On Linux, sudo ip mptcp monitor can monitor the status of MPTCP connections.

On other platforms, this PR will not make any impact currently. (I just tested it on macOS)

[AkinoKaede]

I think the errors reported by Lint are not caused by this PR

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 29.16667% with 34 lines in your changes missing coverage. Please review.

Project coverage is 35.60%. Comparing base (346ca66) to head (7ac7bba).
Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
transport/internet/config.pb.go	3.70%	26 Missing ⚠️
transport/internet/system_listener.go	50.00%	3 Missing and 1 partial ⚠️
infra/conf/cfgcommon/socketcfg/socket.go	71.42%	1 Missing and 1 partial ⚠️
transport/internet/system_dialer.go	66.66%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3109      +/-   ##
==========================================
+ Coverage   35.59%   35.60%   +0.01%     
==========================================
  Files         729      729              
  Lines       41613    41660      +47     
==========================================
+ Hits        14812    14834      +22     
- Misses      25167    25193      +26     
+ Partials     1634     1633       -1     

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[xiaokangwang]

I think this pull request is ready to be merged. Thanks for your contribution.

Documentation pull request for this merge request is welcomed. To reduce time to process merge request, documentation are no longer release blocker.