[vc-authn-oidc] Add proof-request format configuration

Following [this issue](https://github.com/openwallet-foundation/acapy-vc-authn-oidc/issues/903) and the [related PR](https://github.com/openwallet-foundation/acapy-vc-authn-oidc/pull/904), a new setting needs to be added to the chart to allow for the explicit configuration of this environment variable.