feat: refactor external providers dir (#2049)

# What does this PR do?

currently the "default" dir for external providers is
`/etc/llama-stack/providers.d`

This dir is not used anywhere nor created.

Switch to a more friendly `~/.llama/providers.d/`

This allows external providers to actually create this dir and/or
populate it upon installation, `pip` cannot create directories in `etc`.

If a user does not specify a dir, default to this one

see https://github.com/containers/ramalama-stack/issues/36

Signed-off-by: Charlie Doern <cdoern@redhat.com>

tests/external-provider/llama-stack-provider-ollama/custom-distro.yaml

@@ -6,4 +6,4 @@ distribution_spec:
     - remote::custom_ollama
 image_type: container
 image_name: ci-test
-external_providers_dir: /tmp/providers.d
+external_providers_dir: ~/.llama/providers.d

tests/external-provider/llama-stack-provider-ollama/run.yaml

@@ -91,4 +91,4 @@ tool_groups:
   provider_id: wolfram-alpha
 server:
   port: 8321
-external_providers_dir: /tmp/providers.d
+external_providers_dir: ~/.llama/providers.d