feat: add ability to use custom certs for email

With argoproj/argo-cd#15394 it has been possible to use custom certs from
`argocd-tls-certs-cm`. This works as intended for any notification that
is based on HTTP, but _not_ for emails (since the email doesn't make use
of `http.Transport`).

This commit fixes that by extracting out the TLS config to a separate
function which can be called by the email service. The functionality
which creates an email has been extracted from the library which was
previously used since there's no support for setting custom TLS configs.

This also means we can get rid of the `gomodules.xyz/notify` library.

Signed-off-by: Blake Pettersson <[email protected]>

Author: blakepettersson

go.mod

@@ -28,8 +28,8 @@ require (
 	github.com/spf13/cobra v1.9.1
 	github.com/stretchr/testify v1.10.0
 	golang.org/x/time v0.10.0
-	gomodules.xyz/notify v0.1.1
 	google.golang.org/api v0.223.0
+	gopkg.in/gomail.v2 v2.0.0-20160411212932-81ebce5c23df
 	gopkg.in/yaml.v3 v3.0.1
 	k8s.io/api v0.32.2
 	k8s.io/apimachinery v0.32.2
@@ -107,13 +107,11 @@ require (
 	golang.org/x/sys v0.30.0 // indirect
 	golang.org/x/term v0.29.0 // indirect
 	golang.org/x/text v0.22.0 // indirect
-	gomodules.xyz/envconfig v1.3.1-0.20190308184047-426f31af0d45 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20250219182151-9fdb1cabc7b2 // indirect
 	google.golang.org/grpc v1.70.0 // indirect
 	google.golang.org/protobuf v1.36.5 // indirect
 	gopkg.in/alexcesaro/quotedprintable.v3 v3.0.0-20150716171945-2caba252f4dc // indirect
 	gopkg.in/evanphx/json-patch.v4 v4.12.0 // indirect
-	gopkg.in/gomail.v2 v2.0.0-20160411212932-81ebce5c23df // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	k8s.io/klog/v2 v2.130.1 // indirect
 	k8s.io/kube-openapi v0.0.0-20241105132330-32ad38e42d3f // indirect