Config: Can add static headers to email messages (#79365)

* Can add allowed custom headers to an email Message. WIP.

* adds slug as a custom email header to all outgoing emails

* Headers are static - declared as key/value pairs in config. All static headers get added to emails.

* updates comment

* adds tests for parsing smtp static headers

* updates test to assert static headers are included when building email

* updates test to use multiple static headers

* updates test names

* fixes linting issue with error

* ignore gocyclo for loading config

* updates email headers in tests to be formatted properly

* add static headers first

* updates tests to assert that regular headers like From cant be overwritten

* ensures only the header is in a valid format for smtp and not the value

* updates comment and error message wording

* adds to docs and ini sample files

* updates smtp.static_headers docs examples formatting

* removes lines commented with semi colons

* prettier:write

* renames var

pkg/setting/setting.go

@@ -1026,6 +1026,7 @@ func (cfg *Cfg) validateStaticRootPath() error {
 	return nil
 }
 
+// nolint:gocyclo
 func (cfg *Cfg) Load(args CommandLineArgs) error {
 	cfg.setHomePath(args)
 
@@ -1197,7 +1198,9 @@ func (cfg *Cfg) Load(args CommandLineArgs) error {
 	cfg.handleAWSConfig()
 	cfg.readAzureSettings()
 	cfg.readSessionConfig()
-	cfg.readSmtpSettings()
+	if err := cfg.readSmtpSettings(); err != nil {
+		return err
+	}
 	if err := cfg.readAnnotationSettings(); err != nil {
 		return err
 	}