[BE] PMM->Portal: prepare common interfaces - model

User story
To avoid additional changes in code when real Portal API for adding PMM is created and to be able to build our code in https://jira.percona.com/browse/PMM-9063 and https://jira.percona.com/browse/PMM-9060 in parallel, let's create a model for okta params we get from the Portal API.

In addition to that, Let's create an Okta SSO service which is responsible for access_token handling.

Suggested implementation
Store okta sso details in separate table:

• portal_auth_client_id= <ClientId>

• portal_auth_client_secret=<ClientSecret>

• portal_auth_issuer=<ISSUER_URL>

• portal_auth_scope=<Scope>

Details
I don't think it's a good idea to store it in settings table that already exists. One would need to prevent user from getting it like other settings using: