enhance: ask user for OpenAI key and store it in the cred store

[g-linville]

No description provided.

[njhale]

nit:

Suggested change

	oAIClient, err := openai.NewClient(cliCfg, opts.CredentialContext, opts.OpenAI, openai.Options{
	oaiClient, err := openai.NewClient(cliCfg, opts.CredentialContext, opts.OpenAI, openai.Options{

[g-linville]

I made some pretty substantial changes to this PR:

• We now use SysPrompt to ask the user (which means I had to move it to its own package in order to avoid an import cycle)
• We only prompt the user for the token when it is actually needed
  • This involved creating a custom error type and checking for it in a couple places, which is a bit gross but is the cleanest solution I could come up with for this

[cjellick]

We will need to make sure this works with @tylerslaton's UI, once he has prompting support

[g-linville]

Okay, this is finally ready again. Darren wanted me to add support for the remote client to prompt for credentials as well, so that is the main thing I introduced. I also had to make some changes to accommodate the changes in the prompt server.