Xray-core "MitM" requires a certificate to run

[patterniha]

XRay-core will unveil MitM in next version (MitM + Domain Fronting).

the users need to create and import a self-signed-certificate into the config:

"certificates": [
            {
              "usage": "issue",
              "certificateFile": "mycert.crt",
              "keyFile": "mycert.key"
            }
]

so like geo-asset-files we need a section in v2rayNG to import the user certificate and its private-key.

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also, Xray-core can generate self-signed-certificate with xray tls cert -ca -file=mycert command,

this command generate a certificate and its private-key and saves them in: mycert.crt and mycert.key

in addition, the users need to import mycert.crt into the android-system,

So In addition to the ability to Import the existing user certificate, v2rayNG must be able to generate and export a new certificate.