chore: tweak .clang-tidy

1. Increase cognitive complexity threshold to 50. The default value of
   25 marks a lot of functions, which is too noisy to be useful.
2. Ignore int8_t in signed char to integer conversion check.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

.clang-tidy

@@ -23,5 +23,9 @@ CheckOptions:
     value: 255.0;1.0;
   - key: readability-function-cognitive-complexity.IgnoreMacros
     value: true
+  - key: readability-function-cognitive-complexity.Threshold
+    value: 50
   - key: readability-function-cognitive-complexity.DescribeBasicIncrements
     value: true
+  - key: bugprone-signed-char-misuse.CharTypdefsToIgnore
+    value: int8_t