Add systemd service unit file

[tacerus]

This imports a unit file allowing users to run mtail as a system service on systemd based systems.

Submitted on behalf of a third-party: openSUSE contributors

Contributed from https://build.opensuse.org/package/view_file/server:monitoring/mtail/mtail.service as per the discussion in https://github.com/google/mtail/issues/314.

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Unit Test Results

       1 files  ±0       27 suites  ±0   9m 9s :stopwatch: +23s    645 tests ±0     644 :heavy_check_mark: ±0  1 :zzz: ±0  0 :x: ±0  1 908 runs  ±0  1 905 :heavy_check_mark: ±0  3 :zzz: ±0  0 :x: ±0 

Results for commit dd0f4dae. ± Comparison against base commit def654e6.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.76%. Comparing base (2a4c624) to head (dd0f4da). Report is 17 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #819 +/- ## ======================================= Coverage 56.76% 56.76% ======================================= Files 66 66 Lines 6923 6923 ======================================= Hits 3930 3930 Misses 2724 2724 Partials 269 269 ```

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Unit Test Results

       1 files  ±0       27 suites  ±0   8m 59s :stopwatch: +13s    645 tests ±0     644 :heavy_check_mark: ±0  1 :zzz: ±0  0 :x: ±0  1 908 runs  ±0  1 905 :heavy_check_mark: ±0  3 :zzz: ±0  0 :x: ±0 

Results for commit add580b2. ± Comparison against base commit def654e6.