PKI ed25519 key type response in PKCS8 format fails

[navaneethrameshan]

Describe the bug
Generating an ed25519 key with the PKI plugin and expecting the key in PKCS8 format fails with error:

* unknown private key type "ed25519"

To Reproduce
Steps to reproduce the behavior:

1. Run

vault write pki/root/generate/exported \ 
common_name="example.com" \
key_type="ed25519" \
private_key_format="pkcs8"

2. See error:

Error writing data to pki/root/generate/exported: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/pki/root/generate/exported
Code: 500. Errors:

* 1 error occurred:
	* unknown private key type "ed25519"

Expected behavior
Expect root CA certificate to be generated and the key generation to succeed

Environment:

• Vault Server Version (retrieve with vault status):

Key             Value
---             -----
Seal Type       shamir
Initialized     true
Sealed          false
Total Shares    1
Threshold       1
Version         1.10.0-dev
Storage Type    inmem
Cluster Name    vault-cluster-483668ff
Cluster ID      cca902b2-049e-67a1-6009-647327ba5daf
HA Enabled      false

• Vault CLI Version (retrieve with vault version):

Vault v1.10.0-dev ('7800c45bb7a968757fb18264bee4ef5f2517e62c+CHANGES')

[stevendpclark]

Closing issue, thanks to @navaneethrameshan for the PR contribution!

The fix was also backported to 1.9 within #13276 and should be available with the next 1.9 release.