add support for configuring uv in Workbench

[brooklynbagel]

The Helm chart currently doesn't have a way to configure /etc/uv/uv.toml or $XDF_CONFIG_DIRS/uv/uv.toml in order to set the uv index url: https://docs.posit.co/ide/server-pro/admin/python/package_installation.html#configuring-additional-pip-and-uv-settings and https://docs.astral.sh/uv/concepts/configuration-files/. This attempts to resolve the situation by adding support for a config.session.uv\.toml similar to config.session.pip\.conf.

Note: I left this as a draft due to two open questions:

1. Should a helper for .toml be added to https://github.com/rstudio/helm/blob/main/charts/rstudio-library/templates/_config.tpl like we have for .ini, .gcfg, .json?
2. An potentially lighter weight solution would just be to update the Helm chart examples to instruct users to set the UV_INDEX_URL environment variable for session pods via our example config: https://docs.posit.co/helm/examples/workbench/application-configuration/ uv already supports a range of configuration via environment variables: https://docs.astral.sh/uv/reference/environment/

[CLAassistant]

All committers have signed the CLA.