fix(relay)!: change default cert format from der to pem (#3204)

## Description

PEM format is probably the more used option for this use case so
switching around defaults. ~~Also adding `cert_format` so you can
specify `DER` as an alternative.~~

## Breaking Changes

Changes the default cert format for `Reloading` certificate mode from
`DER` to `PEM`

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
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## Change checklist
<!-- Remove any that are not relevant. -->
- [ ] Self-review.
- [ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.
- [ ] List all breaking changes in the above "Breaking Changes" section.
- [ ] Open an issue or PR on any number0 repos that are affected by this
breaking change. Give guidance on how the updates should be handled or
do the actual updates themselves. The major ones are:
    - [ ] [`quic-rpc`](https://github.com/n0-computer/quic-rpc)
    - [ ] [`iroh-gossip`](https://github.com/n0-computer/iroh-gossip)
    - [ ] [`iroh-blobs`](https://github.com/n0-computer/iroh-blobs)
    - [ ] [`dumbpipe`](https://github.com/n0-computer/dumbpipe)
    - [ ] [`sendme`](https://github.com/n0-computer/sendme)

Author: Arqu

iroh-relay/src/main.rs

@@ -309,7 +309,7 @@ struct TlsConfig {
     ///
     /// Defaults to the servers' current working directory.
     cert_dir: Option<PathBuf>,
-    /// Path of where to read the certificate from for the `Manual` `cert_mode`.
+    /// Path of where to read the certificate from for the `Manual` and `Reloading` `cert_mode`.
     ///
     /// Defaults to `<cert_dir>/default.crt`.
     ///
@@ -534,11 +534,11 @@ async fn maybe_load_tls(
 
             let key_reader = rustls_cert_file_reader::FileReader::new(
                 key_path,
-                rustls_cert_file_reader::Format::DER,
+                rustls_cert_file_reader::Format::PEM,
             );
             let certs_reader = rustls_cert_file_reader::FileReader::new(
                 cert_path,
-                rustls_cert_file_reader::Format::DER,
+                rustls_cert_file_reader::Format::PEM,
             );
 
             let loader: CertifiedKeyLoader<