Add accessor for user-agent header

[jhg03a]

Describe the feature request or question

I'd like to see something like the WithToken / SetToken ability with regards to setting the user-agent string. It seems at the moment this is hardcoded to specific library information, but I'd like to set it to my own app version info so that I can distinguish clients on different builds of my application that share the same vault library versions

Link to the Vault API Docs that support this feature

Not Applicable

Additional context