fallback to env variable for oidc,oauth2 client id and secret

[czhou-brex]

Similar to secureOauthTokenintrospectionAllKV, support falling back to environment varible for client id and secret for oidc and oauth2 filters, when the values passed in are empty strings.

This will allow compatibility with external secret stores.

Testing:

• Run command line skipper with empty args for client_id and/or client_secret, in combination with those values set via env vars in oidc filter
• Run command line skipper with empty args for client_id and/or client_secret, in combination with those values set via env vars in oauthgrant filter

[czhou-brex]

Possibly closes #1952

[bjhaid]

cc: @szuecs please help review 🙏

[szuecs]

we should lookup once in skipper.go and pass the result values via OidcOptions

[szuecs]

see also https://github.com/zalando/skipper/blob/master/skipper.go#L1674-L1680

[czhou-brex]

The existing OidcOptions struct does not have OIDC_Client_ID - should that be added to the field as a struct?
In addition would the arguments parsing in oidc.go have to be updated? https://github.com/zalando/skipper/blob/master/filters/auth/oidc.go#L80

Edit: I have added the two fields as structs and fetch the env variables in skipper.go

[szuecs]

To close #1952 we would need to use secrets module and read from file to allow secrets rotation.

[szuecs]

Changes look good. Please update the docs for the filters: see docs/reference/filters.md

table in https://opensource.zalando.com/skipper/reference/filters/#oauth2
listing in https://opensource.zalando.com/skipper/reference/filters/#openid-connect

For CLI flags we would override CLI values with the ENV, which should be written in help messages:
https://github.com/zalando/skipper/blob/master/config/config.go#L484-L485

[czhou-brex]

Changes look good. Please update the docs for the filters: see docs/reference/filters.md

table in https://opensource.zalando.com/skipper/reference/filters/#oauth2 listing in https://opensource.zalando.com/skipper/reference/filters/#openid-connect

For CLI flags we would override CLI values with the ENV, which should be written in help messages: https://github.com/zalando/skipper/blob/master/config/config.go#L484-L485

Docs have been updated.

[szuecs]

👍

[RomanZavodskikh]

👍

[MustafaSaber]

👍

[MustafaSaber]

thanks @czhou-brex for your contribution 🙏!