[Openid-specs-fapi] Issue #458: FAPI1 Part1: not clear as to which auth flows are supported (openid/fapi)

[Kosuke Koiwai]

New issue 458: FAPI1 Part1: not clear as to which auth flows are supported
https://bitbucket.org/openid/fapi/issues/458/fapi1-part1-not-clear-as-to-which-auth

Kosuke Koiwai:

In Issue #11, Nat says “PKCE or Hybrid Flow is mandated in Part 1,“ but I couldn’t read it from the spec.

Does “shall support PCKE” means “shall not use implicit flow?” 

Part2 has been already covered in Issue #72.

### 5.2.3.  Public client

1. shall support [RFC7636](https://tools.ietf.org/html/rfc7636);
2. shall use `S256` as the code challenge method for the [RFC7636](https://tools.ietf.org/html/rfc7636);

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