Fix to send challenge only if provider supports PKCE

[int128]

This will fix #282.

Verified with GitLab.

% go run main.go setup --oidc-issuer-url=https://gitlab.com --oidc-client-id=REDUCTED
authentication in progress...
error: setup: authentication error: authentication error: authorization code flow error: oauth2 error: could not exchange the code and token: oauth2: cannot fetch token: 401 Unauthorized
Response: {"error":"invalid_request","error_description":"The request is missing a required parameter, includes an unsupported parameter value, or is otherwise malformed."}
exit status 1

% go run main.go setup --oidc-issuer-url=https://gitlab.com --oidc-client-id=REDUCTED
authentication in progress...

## 2. Verify authentication

You got a token with the following claims:

{
  "iss": "https://gitlab.com",
...

[codecov-io]

Codecov Report

Merging #283 into master will increase coverage by 2.08%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   70.31%   72.40%   +2.08%     
==========================================
  Files          20       20              
  Lines         721      714       -7     
==========================================
+ Hits          507      517      +10     
+ Misses        149      130      -19     
- Partials       65       67       +2     

Impacted Files	Coverage Δ
pkg/domain/pkce/pkce.go	85.18% <85.18%> (ø)
pkg/usecases/authentication/authcode.go	63.26% <100.00%> (-1.45%)	⬇️
pkg/usecases/authentication/authcode_keyboard.go	71.42% <100.00%> (-0.80%)	⬇️

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