Add python-downloads-json-url option for uv.toml to configure custom Python installations via JSON URL (#12974)

## Summary

Part of #12838. Allow users to configure `python-downloads-json-url` in
`uv.toml` and not just from env.

I followed similar PR #8695, so same as there it's also available in the
CLI (I think maybe it's better not to be configurable from the CLI, but
since the mirror parameters are, I think it's better to do the same)


## Test Plan

<!-- How was it tested? -->

docs/reference/cli.md

@@ -4770,6 +4770,11 @@ uv python list [OPTIONS] [REQUEST]
 <p>This setting has no effect when used in the <code>uv pip</code> interface.</p>
 
 <p>May also be set with the <code>UV_PROJECT</code> environment variable.</p>
+</dd><dt id="uv-python-list--python-downloads-json-url"><a href="#uv-python-list--python-downloads-json-url"><code>--python-downloads-json-url</code></a> <i>python-downloads-json-url</i></dt><dd><p>URL pointing to JSON of custom Python installations.</p>
+
+<p>Note that currently, only local paths are supported.</p>
+
+<p>May also be set with the <code>UV_PYTHON_DOWNLOADS_JSON_URL</code> environment variable.</p>
 </dd><dt id="uv-python-list--quiet"><a href="#uv-python-list--quiet"><code>--quiet</code></a>, <code>-q</code></dt><dd><p>Use quiet output.</p>
 
 <p>Repeating this option, e.g., <code>-qq</code>, will enable a silent mode in which uv will write no output to stdout.</p>
@@ -4941,6 +4946,11 @@ uv python install [OPTIONS] [TARGETS]...
 <p>Distributions can be read from a local directory by using the <code>file://</code> URL scheme.</p>
 
 <p>May also be set with the <code>UV_PYPY_INSTALL_MIRROR</code> environment variable.</p>
+</dd><dt id="uv-python-install--python-downloads-json-url"><a href="#uv-python-install--python-downloads-json-url"><code>--python-downloads-json-url</code></a> <i>python-downloads-json-url</i></dt><dd><p>URL pointing to JSON of custom Python installations.</p>
+
+<p>Note that currently, only local paths are supported.</p>
+
+<p>May also be set with the <code>UV_PYTHON_DOWNLOADS_JSON_URL</code> environment variable.</p>
 </dd><dt id="uv-python-install--quiet"><a href="#uv-python-install--quiet"><code>--quiet</code></a>, <code>-q</code></dt><dd><p>Use quiet output.</p>
 
 <p>Repeating this option, e.g., <code>-qq</code>, will enable a silent mode in which uv will write no output to stdout.</p>

docs/reference/settings.md

@@ -1655,6 +1655,32 @@ Whether to allow Python downloads.
 
 ---
 
+### [`python-downloads-json-url`](#python-downloads-json-url) {: #python-downloads-json-url }
+
+URL pointing to JSON of custom Python installations.
+
+Note that currently, only local paths are supported.
+
+**Default value**: `None`
+
+**Type**: `str`
+
+**Example usage**:
+
+=== "pyproject.toml"
+
+    ```toml
+    [tool.uv]
+    python-downloads-json-url = "/etc/uv/python-downloads.json"
+    ```
+=== "uv.toml"
+
+    ```toml
+    python-downloads-json-url = "/etc/uv/python-downloads.json"
+    ```
+
+---
+
 ### [`python-install-mirror`](#python-install-mirror) {: #python-install-mirror }
 
 Mirror URL for downloading managed Python installations.