Provider ABI for IExecutionProvider OnnxRuntime extension infrastructure

Is your feature request related to a problem? Please describe.

Currently all OnnxRuntime -backend execution plugins (IExecutionProvider implementations) require static linking as well as compiling onnxruntime as a submodule or with the same compiler and distributing a package with Onnxruntime statically linked with the plugin.

Providing an ABI not only allows for linking against pre-built onnxruntime binaries. It also allows for a leaner onnxruntime codebase as well as more freedom into the way an onnx session can operate.

It also makes the life of a onnxruntime-backend-plugin team easier since they can now only ship a runtime plugin for the standard onnxruntime package.

Describe the solution you'd like
All APIs currently required for implementing an Execution Provider should be based off an ABI underneath to allow for out-of-module linking with OnnxRuntime without needing a code change to the Provider code

Describe alternatives you've considered
The alternative is the quite frustrating experience of compiling the entire onnxruntime and distributing a single onnxruntime-custom plugin wheel instead of simply distributing a runtime plugin for onnxruntime to be loaded dynamically.

Additional context

The issue names IExecutionProvider implementations and dynamically loaded plugins, but no files, tests, or entry points. Start by locating the existing Execution Provider API and its plugin-loading path, then determine the ABI boundary and compatibility requirements. Done would mean a documented, testable provider ABI that supports out-of-module linking without provider code changes.