Python: Link `stubgen.{py,sh}` to `resources/` and read them using `getResource()` (#2692)

## Motivation and Context
Currently, we're relying on `runtimeConfig.runtimeCratesPath()` to read
`stubgen.{py,sh}` files, but that function only returns a path if we're
using `relativePath` for our runtime crates:
```
"runtimeConfig": {
      "relativePath": "../rust-runtime"
},
```
otherwise it returns `null` and in that case our code generator fails
with:
```
Projection pokemon-service failed: java.io.FileNotFoundException: null/aws-smithy-http-server-python/stubgen.py (No such file or directory)
java.io.FileNotFoundException: null/aws-smithy-http-server-python/stubgen.py (No such file or directory)
```

With this PR we're linking `stubgen.{py,sh}` to `resources/` folder and
reading them using `getResource()` function.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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