refactor: install external providers from module (#2637)

# What does this PR do?

Today, external providers are installed via the `external_providers_dir`
in the config. This necessitates users to understand the `ProviderSpec`
and set up their directories accordingly. This process splits up the
config for the stack across multiple files, directories, and formats.

Most (if not all) external providers today have a
[get_provider_spec](559cb18fbb/src/ramalama_stack/provider.py (L9))
method that sits unused. Utilizing this method rather than the
providers.d route allows for a much easier installation process for
external providers and limits the amount of extra configuration a
regular user has to do to get their stack off the ground.

To accomplish this and wire it throughout the build process, Introduce
the concept of a `module` for users to specify for an external provider
upon build time. In order to facilitate this, align the build and run
spec to use `Provider` class rather than the stringified provider_type
that build currently uses.

For example, say this is in your build config:

```
- provider_id: ramalama
  provider_type: remote::ramalama
  module: ramalama_stack
```

during build (in the various `build_...` scripts), additionally to
installing any pip dependencies we will also install this module and use
the `get_provider_spec` method to retrieve the ProviderSpec that is
currently specified using `providers.d`.

In production so far, providing instructions for installing external
providers for users has been difficult: they need to install the module
as a pre-req, create the providers.d directory, copy in the provider
spec, and also copy in the necessary build/run yaml files. Accessing an
external provider should be as easy as possible, and pointing to its
installable module aligns more with the rest of our build and dependency
management process.

For now, `external_providers_dir` still exists as an alternate more
declarative method of using external providers.

## Test Plan

added an integration test installing an external provider from module
and more unit test coverage for `get_provider_registry`


( the warning in yellow is expected, the module is installed inside of
the build env, not where we are running the command)
<img width="1119" height="400" alt="Screenshot 2025-07-24 at 11 30
48 AM"
src="https://github.com/user-attachments/assets/1efbaf45-b9e8-451a-bd63-264ed664706d"
/>

<img width="1154" height="618" alt="Screenshot 2025-07-24 at 11 31
14 AM"
src="https://github.com/user-attachments/assets/feb2b3ea-c5dd-418e-9662-9a3bd5dd6bdc"
/>

---------

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

tests/external/ramalama-stack/build.yaml

@@ -0,0 +1,14 @@
+version: 2
+distribution_spec:
+  description: Use (an external) Ramalama server for running LLM inference
+  container_image: null
+  providers:
+    inference:
+    - provider_id: ramalama
+      provider_type: remote::ramalama
+      module: ramalama_stack==0.3.0a0
+image_type: venv
+image_name: ramalama-stack-test
+additional_pip_packages:
+- aiosqlite
+- sqlalchemy[asyncio]

tests/external/ramalama-stack/run.yaml

@@ -0,0 +1,12 @@
+version: 2
+image_name: ramalama
+apis:
+- inference
+providers:
+  inference:
+  - provider_id: ramalama
+    provider_type: remote::ramalama
+    module: ramalama_stack==0.3.0a0
+    config: {}
+server:
+  port: 8321