Allow APNS key to be in unencrypted PKCS8 format (#24570)

#23760 

Manual QA done:
1. Get the current APNS key using tools/mdm/assets tool.
2. Convert it to a PKCS8 key like: `openssl pkcs8 -topk8 -inform PEM
-outform PEM -in mdm_assets/apns_key.key -out mdm_assets/apns_pkcs8.key
-nocrypt`
3. Delete all `apns` and `scep` rows from `mdm_config_assets` DB table.
4. Point to the PKCS8 key like:

```
export FLEET_MDM_APPLE_APNS_KEY=/Users/victor/work/fleet/mdm_assets/apns_pkcs8.key

export FLEET_MDM_APPLE_APNS_CERT=/Users/victor/work/fleet/mdm_assets/apns_cert.crt
export FLEET_MDM_APPLE_SCEP_CERT=/Users/victor/work/fleet/mdm_assets/ca_cert.crt
export FLEET_MDM_APPLE_SCEP_KEY=/Users/victor/work/fleet/mdm_assets/ca_key.key
export FLEET_MDM_APPLE_SCEP_CHALLENGE=$(cat /Users/victor/work/fleet/mdm_assets/scep_challenge)
```

This step may be tricky. Might be simpler to spin up a fresh Fleet
server with the SCEP/APNS values.

5. Start fleet server. Make sure APNS works by pushing/deleting a
profile.
6. Renew APNS cert and make sure the new cert still works.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

<!-- Note that API documentation changes are now addressed by the
product design team. -->

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
- [x] Added/updated tests
- [x] Manual QA for all new/changed functionality

server/service/testdata/pkcs1.key

@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
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+-----END RSA PRIVATE KEY-----

server/service/testdata/pkcs8-encrypted.key

@@ -0,0 +1,18 @@
+-----BEGIN ENCRYPTED PRIVATE KEY-----
+MIIC3TBXBgkqhkiG9w0BBQ0wSjApBgkqhkiG9w0BBQwwHAQIkErtXjGCalMCAggA
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+UPRGHqltz4uOjbD1epkSGe0=
+-----END ENCRYPTED PRIVATE KEY-----

server/service/testdata/pkcs8-rsa.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
+MIIEvgIBADANBgkqhkiG9w0BAQEFAASCBKgwggSkAgEAAoIBAQCV9Y99iSEwRvKF
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+-----END PRIVATE KEY-----

server/service/testdata/pkcs8-x25519.key

@@ -0,0 +1,3 @@
+-----BEGIN PRIVATE KEY-----
+MC4CAQAwBQYDK2VuBCIEIIBy1s9qwcgQqJsrkOeQ9W5smjPait9cF8LHDcYQt/BK
+-----END PRIVATE KEY-----