chore: Refactor Sample Code to Separate Files

Split the code in `source.go` to individual sample files, one per rule.
This will help contributors submit samples for new rules, or
improvements to existing rules. The cgo sample was all that was left
after refactoring, which resulted in its own sample file.

Sample code was also formatted to have some level of consistency.
Each sample go "file" attempts to keep the formatting of `gofmt`, and
each code sample is in its own section in the sample file.

Signed-off-by: Adam Kaplan <adam@adambkaplan.com>

testutils/sample_types.go

@@ -0,0 +1,10 @@
+package testutils
+
+import "github.com/securego/gosec/v2"
+
+// CodeSample encapsulates a snippet of source code that compiles, and how many errors should be detected
+type CodeSample struct {
+	Code   []string
+	Errors int
+	Config gosec.Config
+}