Add Scope value predefined in the specification (profile, email, address, phone)

[arukiidou]

Summary

• Add predefined scope values as const - included in the OpenID Connect Core 1.0 specs
  • Add a description docs to each const
  • Update example

If I open this PR, is it acceptable?

Citation of specifications

https://openid.net/specs/openid-connect-core-1_0.html

5.4.  Requesting Claims using Scope Values
profile
OPTIONAL. This scope value requests access to the End-User's default profile Claims, which are: name, family_name, given_name, middle_name, nickname, preferred_username, profile, picture, website, gender, birthdate, zoneinfo, locale, and updated_at.
email
OPTIONAL. This scope value requests access to the email and email_verified Claims.
address
OPTIONAL. This scope value requests access to the address Claim.
phone
OPTIONAL. This scope value requests access to the phone_number and phone_number_verified Claims.

Key changes.

• Omit examples and document updates yet - if you favor this proposal, I will start.

const (
	ScopeProfile = "profile"
	ScopeEmail = "email"
	ScopeAddress = "address"
	ScopePhone = "phone"
)

Anticipated Questions

• Should this be in golang.org/x/oauth2?
  • No. OAuth does not define any particular values for scopes, since it is highly dependent on the service's internal architecture and needs.
• What are the benefits? Why do it?
  • Help users understand OIDC and go-oidc faster.
  • "openid profile email" is a frequently used pattern in OIDC authentication

        // before
	config := oauth2.Config{
		Scopes:       []string{oidc.ScopeOpenID, "profile", "email"},
	}
        // after
	config := oauth2.Config{
		Scopes:       []string{oidc.ScopeOpenID, oidc.ScopeProfile, oidc.ScopeEmail},
	}

[ericchiang]

Yeah this sounds reasonable, feel free to send a PR!

[arukiidou]

@ericchiang

• Thanks! I did it #387