CEP-17 add language-agnostic plugins

cep-17.md

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+<table>
+<tr><td> Title </td><td> Language-Independent Plugins</td>
+<tr><td> Status </td><td> Draft </td></tr>
+<tr><td> Author(s) </td><td> Matthew R. Becker &lt;[email protected]&gt;</td></tr>
+<tr><td> Created </td><td> May 31, 2024</td></tr>
+<tr><td> Updated </td><td> May 31, 2024</td></tr>
+<tr><td> Discussion </td><td> NA </td></tr>
+<tr><td> Implementation </td><td> NA </td></tr>
+</table>
+
+## Abstract
+
+This CEP proposes a standard for language-independent plugins for `conda` and `conda`-like tools (e.g., `pixi`, `mamba`, `rattler`). 
+The goal is to generalize the existing work on plugins in `conda` via `pluggy` to something that can be implemented 
+by more parties in the ecosystem. The standard below augments the existing `pluggy` plugins by defining an additional 
+source of plugin information that `conda` can use to generate plugins for `pluggy`. Other `conda`-compliant package managers 
+can use the same information to take appropriate actions (e.g., add a virtual package to the repodata) in order to achieve 
+the same effects. The primary target of this CEP is virtual packages. Other possible cross-language plugins can be added in future 
+CEPS as the need arises.
+
+## Specification
+
+To generate plugins, a `conda`-compliant tool will 
+
+1. Look for the `conda-plugins` executable on the current PATH.
+2. If `conda-plugins` is found, execute it.
+3. If `conda-plugins` executes successfully, parse `stdout` as JSON and take the appropriate action as defined below.
+4. If `conda-plugins` executes non-successfully, display an error to `stderr` containing any `stderr` from `conda-plugins` and continue.
+
+The JSON output from `conda-plugins` must be formatted and interpreted as follows:
+
+```JSON
+{
+  "virtual_pkgs": [
+    {
+      "name": "pkg1-name",
+      "version": "pkg1-version",
+      "build": "pkg1-build",
+    },
+    {
+      "name": "pkg2-name",
+      "version": "pkg2-version",
+      "build": "pkg2-build",
+    }
+  ]
+}
+```
+
+### `virtual_pkgs`
+
+For every item in the `virtual_pkgs` list, a virtual package is defined with name `__<name>`, version `<version>`, and build string `<build>`. 
+Any errors must be printed to `stderr` and the tool must continue. If the list is empty, no virtual packages will be defined. The `virtual_pkgs` key 
+must be present at the top-level.
+
+## Backwards Compatibility
+
+The specification in this CEP can easily be made backwards compatible with the existing `conda` plugin architecture. 
+A PR to `conda` should be made to run the `conda-plugins` executable, if it exists, and take the appropriate actions. 
+In order to ensure conisstent behavior across the ecosystem, any plugins derived from `conda-plugins` should override 
+any other source of plugins, including plugins implemented via `pluggy`.
+
+## Copyright
+
+All CEPs are explicitly [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).