Embed unimplemented server by value instead of pointer

[shuheiktgw]

Since grpc/grpc-go#7438 was merged, our unit tests started failing since we embeded the unimplemented servers by pointer. To fix the problem, use value instead.

[github-actions]

Code Metrics Report

	main (98f250e)	#219 (c4e1515)	+/-
Coverage	65.2%	65.2%	0.0%
Code to Test Ratio	1:0.3	1:0.3	0.0
Test Execution Time	7m11s	7m11s	0s

Details

  |                     | main (98f250e) | #219 (c4e1515) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          65.2% |          65.2% | 0.0% |
  |   Files             |             69 |             69 |    0 |
  |   Lines             |          11756 |          11756 |    0 |
  |   Covered           |           7662 |           7662 |    0 |
  | Code to Test Ratio  |          1:0.3 |          1:0.3 |  0.0 |
  |   Code              |          37082 |          37082 |    0 |
  |   Test              |          12653 |          12653 |    0 |
  | Test Execution Time |          7m11s |          7m11s |   0s |

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Reported by octocov