feat(agnoster): add VRF name to prompt if available

[Pelt10]

Standards checklist:

• [x] The PR title is descriptive.
• [x] The PR doesn't replicate another PR which is already open.
• [x] I have read the contribution guide and followed all the instructions.
• [x] The code follows the code style guide detailed in the wiki.
• [x] The code is mine or it's from somewhere with an MIT-compatible license.
• [x] The code is efficient, to the best of my ability, and does not waste computer resources.
• [x] The code is stable and I have tested it myself, to the best of my abilities.
• [x] If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• add VRF name to prompt if available("ip" command available and a vrf is actually in use)

[bdwalton]

This looks technically fine (to me), but I wonder if it is something that everyone (even in a VRF environment) would want? Also, would it make sense to augment the context section with this instead? Maybe not, but asking.