add keepalive for vpn tunnel

[codecov[bot]]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (44173d7) 36.12% compared to head (e154dca) 35.92%. Report is 1 commits behind head on main.

Files	Patch %	Lines
pkg/networkengine/vpndriver/libreswan/libreswan.go	0.00%	11 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #159 +/- ## ========================================== - Coverage 36.12% 35.92% -0.21% ========================================== Files 12 12 Lines 1431 1439 +8 ========================================== Hits 517 517 - Misses 814 821 +7 - Partials 100 101 +1 ``` | [Flag](https://app.codecov.io/gh/openyurtio/raven/pull/159/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/openyurtio/raven/pull/159/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio) | `35.92% <0.00%> (-0.21%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio#carryforward-flags-in-the-pull-request-comment) to find out more.

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