Support public OIDC clients without the client-secret defined

### Description

As of now vault throwing `* both 'oidc_client_id' and 'oidc_client_secret' must be set for OIDC` error message if `oidc_client_secret` was not set. Most probably, a new additional flag would be required to define public OIDC client behavior.

### Current behavior

Working

```
vault write auth/oidc/config \
bound_issuer="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_url="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_ca_pem=@tls/ca.pem \
oidc_client_id='vault' \
oidc_client_secret='vault' \
default_role='oidc-reader'
```

Not working

```
vault write auth/oidc/config \
bound_issuer="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_url="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_ca_pem=@tls/ca.pem \
oidc_client_id='vault' \
default_role='oidc-reader'
```

### Expected behavior

Both command should finish OIDC method configuration without an error

```
vault write auth/oidc/config \
bound_issuer="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_url="https://${MACHINE_IP}:5556/dex" \
oidc_discovery_ca_pem=@tls/ca.pem \
oidc_client_id='vault' \
oidc_client_public=yes \
default_role='oidc-reader'
```

### Misc

Error happens at

https://github.com/hashicorp/vault-plugin-auth-jwt/blob/master/path_config.go#L200

As a workaround I was able to configure OIDC backend with the random gibberish supplied into `oidc_client_secret` parameter, but during the authentication process it failed with

```
Error exchanging oidc code: "oauth2: cannot fetch token: 401 Unauthorized\nResponse: {\"error\":\"invalid_client\",\"error_description\":\"Invalid client credentials.\"}"
```

In the same time a plain HTTP API request with curl and only `client_id` specified worked as expected, provided me Bearer token