[mtls] make some types and functions public

[mtls] make some types and functions public

Make public the necessary functions and types to allow other packages to register identity and roots providers.

- Types `CertificatePrimitive` and `RootsPrimitive`
- Driver registration functions: `RegisterIdentityDriver`, `RegisterRootDriver`
- `newDialContextFunc` -> `MakeDialContextFunc`
- `newTlsCertificate` -> `MakeTlsCertificate`