In the context of Teleport Discover we are trying to ease the usage of Teleport for the user's first interaction.
When adding a new database resource the user must, among other things, generate the mTLS files
Examples:
https://goteleport.com/docs/database-access/guides/postgres-self-hosted/#step-25-create-a-certificatekey-pairhttps://goteleport.com/docs/database-access/guides/mysql-self-hosted/#step-24-create-a-certificatekey-pair
This PR aims to reduce this friction: the user should be able to setup the resource without prior setup of local tools (`tsh login`)
We're doing this by providing an endpoint that will return those exact files
Demo
```shell
marco@lenix ~/p/downloadmtls> curl --silent --insecure 'https://127.0.0.1.nip.io:3080/v1/webapi/sites/lenix/sign' --dat
a '{"hostname":"discover.example.com", "ttl":"9999h", "format": "db"}' --header 'Authorization: Bearer 308bf3dd3019ddc4
2cff44a48e028480' --header 'Content-Type: application/json' -OJ
marco@lenix ~/p/downloadmtls> tar -xvf teleport_mTLS_discover.example.com.tar.gz
server.key
server.crt
server.cas
marco@lenix ~/p/downloadmtls> head -1 server.*
==> server.cas <==
-----BEGIN CERTIFICATE-----
==> server.crt <==
-----BEGIN CERTIFICATE-----
==> server.key <==
-----BEGIN RSA PRIVATE KEY-----
```
Fixes https://github.com/gravitational/teleport/issues/14049
Jakub Nyckowski