PKCE (RFC7636) Support for OAuth

[mweinelt]

• I have read this document: https://miniflux.app/opinionated.html#feature-request

Thanks for developing miniflux and providing OAuth2.0 and OIDC support.

The OAuth2.0 interfaces specified in RFC6749 is susceptible to an authorization code attack,
when the client is hosted publicly.

This is the reason RFC7636 was created, which provices PKCE, that protects against CSRF and authorization code injections.

The golang.org/x/oauth2 package seems to support PKCE, but it would need to be hooked up.

• https://www.oauth.com/oauth2-servers/pkce/ describes Authorization Requests and Code Exchanges
• https://kanidm.github.io/kanidm/stable/integrations/oauth2.html#extended-options-for-legacy-clients is my IDM, which warns about disabling PKCE

cc @pmarschik who already implemented OIDC support in #583.

[fguillot]

PKCE has been added in PR #2048. Tested with various IDM including Kanidm.

https://miniflux.app/docs/howto.html#oauth2