refactor(internal/constant): add RelayType

[ioogle]

Issue: #345

[ioogle]

Since relay types can be specified via the command line, the validateType() function cannot be eliminated. However, it is best practice to use a self-defined type.

[Ehco1996]

@ioogle thanks for your pr, but ci failed, could your please check and fix these errors?

Error: internal/transporter/ws.go:35:25: cannot use cfg.TransportType (variable of type "github.com/Ehco1996/ehco/internal/constant".RelayType) as string value in argument to zap.S().Named (typecheck)

By the way, you can execute make lint and make test commands to check if your patch can pass the CI tests

[Ehco1996]

lgtm