Use system certs by default for oauth2 gitlab calls

[hightoxicity]

If no httpClient is defined for gitlab connector, create one with system trusted ca bundle.

[bonifaido]

Hi @hightoxicity, why is this change needed? I'm concerned a bit, that client authentication by certificates is not used in this case.

[hightoxicity]

Hi guy, our gitlab instance is presenting a self-signed certificate (signed under our internal root CA), dex must trust the root CA used to issue the gitlab certificate, I have not found any way in dex to provide a trusted cert bundle... Thx

[jenting]

There exists a config tlsClientCA used to specify the root CA.

[hightoxicity]

Afaik tlsClientCA is only used to configure grpc client

[jenting]

Afaik tlsClientCA is only used to configure grpc client

Okay, I was thinking is it better to add tlsClientCA by configuration? Because in this way, the user provides the CA certificate in config like LDAP connector

dex/connector/ldap/ldap.go

Lines 70 to 71 in a901e2f

	// Path to a trusted root certificate file.
	RootCA string `json:"rootCA"`

and

dex/connector/ldap/ldap.go

Lines 76 to 77 in a901e2f

	// Base64 encoded PEM data containing root CAs.
	RootCAData []byte `json:"rootCAData"`