feat: add PKCE support to the openid-connect plugin

[qihaiyan]

Description

As a User, I want to be able to use oAuth2 with PKCE

Fixes #7052

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[spacewander]

Let's refer to:
https://github.com/apache/apisix/blob/master/docs/en/latest/internal/testing-framework.md
https://github.com/zmartzone/lua-resty-openidc/blob/b07330120ffe54dd3fbeac247726b76d0f9dc793/tests/spec/pkce_spec.lua#L21
and write the test for this feature.

[soulbird]

Need more test cases to show the effect of use_pkce set to true and false.

[qihaiyan]

# Failed test 't/plugin/openid-connect.t TEST 9: Access route w/o bearer token and go through the full OIDC Relying Party authentication process. - pattern "[error]" should not match any line in error.log but matches line "2022/07/04 20:45:31 [error] 2009\#2009: *32 [lua] openid-connect.lua:315: phase_func(): OIDC authentication failed: accessing discovery url (http://127.0.0.1:8090/auth/realms/University/.well-known/openid-configuration) failed: connection refused, client: 127.0.0.1, server: localhost, request: \"GET /uri HTTP/1.1\", host: \"127.0.0.1:1984\"" (req 0)

why this error occurs when i run the openid-connect.t unit test, how to resolve it? @spacewander

[tzssangglass]

hi @qihaiyan ,pls merge master branch to your develop branch and fix resolve conflics.

[spacewander]

# Failed test 't/plugin/openid-connect.t TEST 9: Access route w/o bearer token and go through the full OIDC Relying Party authentication process. - pattern "[error]" should not match any line in error.log but matches line "2022/07/04 20:45:31 [error] 2009\#2009: *32 [lua] openid-connect.lua:315: phase_func(): OIDC authentication failed: accessing discovery url (http://127.0.0.1:8090/auth/realms/University/.well-known/openid-configuration) failed: connection refused, client: 127.0.0.1, server: localhost, request: \"GET /uri HTTP/1.1\", host: \"127.0.0.1:1984\"" (req 0)

why this error occurs when i run the openid-connect.t unit test, how to resolve it? @spacewander

I am glad to see your question has been answered in #7052.

[qihaiyan]

Let's refer to: https://github.com/apache/apisix/blob/master/docs/en/latest/internal/testing-framework.md https://github.com/zmartzone/lua-resty-openidc/blob/b07330120ffe54dd3fbeac247726b76d0f9dc793/tests/spec/pkce_spec.lua#L21 and write the test for this feature.

TEST 30 and TEST 31 are added to ut. @spacewander @soulbird