fix: accept JWKS responses with +json content types

[zmstone]

Summary

• accept application/*+json response content types when decoding HTTP JSON payloads
• add a regression test for JWKS responses served as application/jwk-set+json; charset=utf-8
• keep existing application/json and application/jwt handling intact

Verification

• escript /mnt/data/emqx/gwt/260416-sso-support-more-content-type/rebar3 eunit --module=oidcc_provider_configuration_worker_test in /mnt/data/emqx/oidcc
• escript ../../rebar3 eunit --module=oidcc_provider_configuration_worker_test in deps/oidcc from the EMQX checkout
• env PROFILE=emqx-enterprise SUITES=apps/emqx_dashboard_sso/test/emqx_dashboard_sso_oidc_SUITE.erl CASES=t_jwks_content_type_suffix make apps/emqx_dashboard_sso-ct in the EMQX checkout

Notes

• This fixes the application/jwk-set+json; charset=utf-8 JWKS failure reported in EMQX 6.1.0 Docker OIDC SSO Login Fails with provider_not_ready emqx#16834.
• A broader docker-backed apps/emqx_dashboard_sso CT run in EMQX was attempted, but the run was blocked by unrelated environment failures in other suites (quicer/libcrypto.so.1.1 and subsequent dirty suite state), so the focused regression coverage above is the relevant verification for this change.