feat: add oauth config setting to enable/disable pkce extension

[PeterRodenkirchTNG]

Describe the PR
Add Proof Key for Code Exchange Support configurable with the OAuthConfig struct.

Relation issue
#99

Additional context
OAuth2 clients that only support Authorization Code Flow with PKCE do not work with the current feature set of echo-swagger.
There is a flag for it in Swagger UI 3.24.0+: usePkceWithAuthorizationCodeGrant and these small changes fix the problem.

[seilersTNG]

Hello, can you please consider reviewing and merging this PR. I think it is an important security feature that should be part of this library. Thank you. Best, Steffen

[andre-easygo]

The regex in https://github.com/swaggo/echo-swagger/blob/master/swagger.go#L138
needs to be updated to: ^(.*/)([^?]*)?(\?.*)?$

This is because if query parameters are included in the redirectUri, with the current regex they will be matched as the filename and the oauth2-redirect.html file won't be found, since it will be something like oauth2-redirect.html?state=blahblah. Without this change we can't adequately perform the PKCE flow from Swagger UI.

Thank you
cc @PeterRodenkirchTNG @seilersTNG