Add support for reading and writing to the nuget.config via a new command trusted-signers

[PatoBeltran]

As part of the client policies change, the spec for nuget.config changes includes a CLI gesture to interact with the list of trusted signers.

The work for adding this command should include the following:

• Add the ability to add, remove, list and update trusted signers to the TrustedSignersProvider
• Add a provider for performing actions for adding, updating and syncing a trusted signer.
• Expose the actions in a CLI gesture that lets you list, add, remove, update and sync a trusted signer.

[PatoBeltran]

Closing as all PRs have been merged