Core: Add feature flag to skip inherited Authorization header in token request

[raphaelsolarski]

Description

When RESTSessionCatalog is configured with a client_credentials,
initSession() exchanges that credential for token, which becomes the Authorization: Bearer ... header on the catalog session.
If a SessionContext also carries a credential (the pattern used by Trino when iceberg.rest-catalog.session=NONE) the first catalog operation triggers contextualSession() -> fromCredential(), which inherits the catalog parent session headers and forwards existing Authorization header alongside client_id+client_secret in body to the authorization server.

Some OAuth2 authorization servers (in particular Okta) rejects such requests for token which contains both Authorization: Bearer abcs header and client_id+client_secret key pair (please see example how to check it directly on Okta).

This PR is a proposition of filtering out Authorization header from headers inherited from parent session for token call in client credentials flow.
To not break any existing use case scenario the filtering is hidden behind skip-inherited-auth-header-in-token-request feature flag passed as config property to RESTSessionCatalog.

Reproduction of behavior directly on Okta

CLIENT_ID="<CLIENT_ID>"
CLIENT_SECRET="<CLIENT_SECRET>"
OKTA_TOKEN_URL="<https://...okta.com/oauth2/.../v1/token>"
OKTA_SCOPE="datacatalog"

# successful request
ACCESS_TOKEN=$(curl -X POST "$OKTA_TOKEN_URL" \
--header "Accept: application/json" \
--header "Content-Type: application/x-www-form-urlencoded" \
--header "X-Client-Version: Apache Iceberg 1.10.1-e.1 (commit 57448704eb9967abee52da13672326592e9ebe25)" \
--data "grant_type=client_credentials" \
--data "client_id=$CLIENT_ID" \
--data "client_secret=$CLIENT_SECRET" \
--data "scope=$OKTA_SCOPE" | jq -r '.access_token')
# results: 200 + {"token_type":"Bearer","expires_in":3600,"access_token":"...","scope":"datacatalog"}

# subsequent request which inherit Authorization header from session
curl -X POST "$OKTA_TOKEN_URL" \
--header "Authorization: Bearer $ACCESS_TOKEN" \
--header "Accept: application/json" \
--header "Content-Type: application/x-www-form-urlencoded" \
--header "X-Client-Version: Apache Iceberg 1.10.1-e.1 (commit 57448704eb9967abee52da13672326592e9ebe25)" \
--data "grant_type=client_credentials" \
--data "client_id=$CLIENT_ID" \
--data "client_secret=$CLIENT_SECRET" \
--data "scope=$OKTA_SCOPE"

# results:
# 401 + {"errorCode":"invalid_client","errorSummary":"No client credentials found.","errorLink":"invalid_client","errorId":"...","errorCauses":[]}

[ebyhr]

nit: We could use equalsIgnoreCase just in case. Future code could add Authorization header with different casing.

[raphaelsolarski]

good point, fixed

[ebyhr]

This line also passes parent.headers() directly. Is it an oversight or intentional?

[raphaelsolarski]

For now it is intentional - in fromCredential it is obvious that this header should not be passed. But in case of token exchange I'm not sure because there are multiple scenarios how token exchange request can be authenticated. I need a bit of time to reason about it.

[ebyhr]

Can we add this property to OAuth2 auth properties in https://github.com/apache/iceberg/blob/main/docs/docs/catalog-properties.md?

[raphaelsolarski]

ok. Added.