connector/linkedin implements authorization strategy via LinkedIn's
OAuth2 endpoint + profile API.

It doesn't implement RefreshConnector as LinkedIn doesn't provide any
refresh token at all (https://developer.linkedin.com/docs/oauth2, Step 5
— Refresh your Access Tokens) and recommends ordinary AuthCode exchange
flow when token refresh is required.
Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>