KeyCloakConfig does not allow to configure trust for key fetching

### Describe the bug

We need to define CA trust for our adapter when it fetches new public keys. Currently this is possible in an arcane way using environment variables but requires detailed knowledge on how to do that.

A natural way to configure that would be with some sort of “ca:” property in KeyCloakConfig.

### Version

15.0.2-redhat-00001

### Expected behavior

It should be easy to supply trust for key fetch requests.

### Actual behavior

We are using NODE_EXTRA_CA_CERTS instead. It is suboptimal and creates a few problems when running CI/CD and when the node process should have different trust roots for different endpoints.

### How to Reproduce?

_No response_

### Anything else?

_No response_

Start at KeyCloakConfig and trace how key-fetch requests currently obtain trust, including the NODE_EXTRA_CA_CERTS path. Define how a CA setting should be supplied through the configuration and verify that it supports different trust roots without relying on a global environment variable.

Written by the indexing model from the issue text.