BUG: Patch Maintainers Annotations

[gabibguti]

Describe the bug
Some things still need to be addressed in Maintainers Annotation feature (regarding PR #3905):

• In checker/check_result.go, rename IsExempted to Annotations ( ⚠️ Add initial Maintainers Annotation parsing #3905 (comment))
• In config/annotations.go, change IsValidReason to private method as isValidReason ( ⚠️ Add initial Maintainers Annotation parsing #3905 (comment)).
• similarly, these errors can probably be private?

  scorecard/config/config.go

  Lines 28 to 31 in 19f8ae1

  	var (
  	ErrInvalidCheck = errors.New("check is not valid")
  	ErrInvalidReason = errors.New("reason is not valid")
  	)

• Clarify the desired types for annotations. Define if Paths should be under a Reason or under an Annotation and same thing for Probes. We need to understand the better way for users to fill in the annotations ( ⚠️ Add initial Maintainers Annotation parsing #3905 (comment)).
• Define if one invalid check or reason during the config parsing should invalidate the whole config, or if it should be logged and skipped ( ⚠️ Add initial Maintainers Annotation parsing #3905 (comment) and ⚠️ Add initial Maintainers Annotation parsing #3905 (comment)).
• Define if in the case a check is annotated it should not be removed from SARIF result runs. Or, if is working as expected and we should not populate runs if there are no relevant results for that check ( ⚠️ Add initial Maintainers Annotation parsing #3905 (comment)).
• Do we need to pass in checks here? Also this doesn't work if you only run a subset of checks but there's an annotation for a check which isn't being run.

  scorecard/config/config.go

  Lines 73 to 74 in 19f8ae1

  	// Parse reads the configuration file from the repo, stored in scorecard.yml, and returns a `Config`.
  	func Parse(r io.Reader, checks []string) (Config, error) {

• Closing the file with t.Cleanup which is opened here:

  scorecard/config/config_test.go

  Lines 134 to 138 in 19f8ae1

  	r, err := os.Open(tt.configPath)
  	if err != nil {
  	t.Fatalf("Could not open config test file: %s", tt.configPath)
  	}
  	result, err := config.Parse(r, allChecks)

• Make all output format options pointers for consistency? AsJSON2, AsString, etc:

  scorecard/pkg/json.go

  Lines 131 to 134 in 19f8ae1

  	// AsJSON2 exports results as JSON for new detail format.
  	func (r *ScorecardResult) AsJSON2(writer io.Writer,
  	checkDocs docs.Doc, o AsJSON2ResultOption,
  	) error {

• Support more config file locations (e.g. .scorecard.yml, .github/.scorecard.yml, ...)
• Update the config README, it uses outdated yaml tags / values

[gabibguti]

Related to: #1907