cargo: Support updating `vendored sources` if config is defined in `.cargo/config.toml`

Hi Team,

Thanks for this awesome tool.
I have a small request. If project has defined `vendored sources` in `.cargo/config.toml` dependabot could also update vendor sources instead of only updating parent `Cargo.toml`

This would be really nice feature to have.

Thanks in advance.

Read the project's Cargo handling and compare how updates are currently made to Cargo.toml with vendored sources configured in .cargo/config.toml. Trace the existing update path first; done means Dependabot can update the configured vendor sources as well as the parent manifest, with behavior covered by an appropriate test.

Written by the indexing model from the issue text.