Add support for retrieving credentials from `keyring` (#2254)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Adds basic keyring auth support for `uv` commands. Adds clone of `pip`'s
`--keyring-provider subprocess` argument (using CLI `keyring` tool).
See issue: https://github.com/astral-sh/uv/issues/1520
## Test Plan
<!-- How was it tested? -->
Hard to write full-suite unit tests due to reliance on
`process::Command` for `keyring` cli
Manually tested end-to-end in a project with GCP artifact registry using
keyring password:
```bash
➜ uv pip uninstall watchdog
Uninstalled 1 package in 46ms
- watchdog==4.0.0
➜ cargo run -- pip install --index-url https://<redacted>/python/simple/ --extra-index-url https://<redacted>/pypi-mirror/simple/ watchdog
Finished dev [unoptimized + debuginfo] target(s) in 0.18s
Running `target/debug/uv pip install --index-url 'https://<redacted>/python/simple/' --extra-index-url 'https://<redacted>/pypi-mirror/simple/' watchdog`
error: HTTP status client error (401 Unauthorized) for url (https://<redacted>/pypi-mirror/simple/watchdog/)
➜ cargo run -- pip install --keyring-provider subprocess --index-url https://<redacted>/python/simple/ --extra-index-url https://<redacted>/pypi-mirror/simple/ watchdog
Finished dev [unoptimized + debuginfo] target(s) in 0.17s
Running `target/debug/uv pip install --keyring-provider subprocess --index-url 'https://<redacted>/python/simple/' --extra-index-url 'https://<redacted>/pypi-mirror/simple/' watchdog`
Resolved 1 package in 2.34s
Installed 1 package in 27ms
+ watchdog==4.0.0
```
`requirements.txt`
```
#
# This file is autogenerated by pip-compile with Python 3.10
# by the following command:
#
# .bin/generate-requirements
#
--index-url https://<redacted>/python/simple/
--extra-index-url https://<redacted>/pypi-mirror/simple/
...
```
```bash
➜ cargo run -- pip install --keyring-provider subprocess -r requirements.txt
Finished dev [unoptimized + debuginfo] target(s) in 0.19s
Running `target/debug/uv pip install --keyring-provider subprocess -r requirements.txt`
Resolved 205 packages in 23.52s
Built <redacted>
...
Downloaded 47 packages in 19.32s
Installed 195 packages in 276ms
+ <redacted>
...
```
---------
Co-authored-by: Thomas Gilgenast <thomas@vant.ai>
Co-authored-by: Zanie Blue <contact@zanie.dev>
