Add griffin feature flag for setting Llama13B as the default

[bbondy]

• Change the default to Mixtral
• Make it controllable via a feature flag that we can set remotely to make it go to Llama2 13B as the default

[bbondy]

cc @petemill @nullhook

[bbondy]

FYI @petemill: Updated to Mixtral

[petemill]

The feature flags available are:

is_freemium_available - if true then all freemium models are actually freemium, if false then all freemium models are only available to premium users.

default_model - hard-coded value is chat-leo-expanded (mistral). If we disable freemium from the server-side, then we would change this in feature flag to chat-basic.

default_premium_model - hard-coded value is chat-leo-expanded. We would probably just keep this the same.

[kjozwiak]

The above requires 1.62.147 or higher for 1.62.x verification 👍 STR/Cases can be found via the verifications that @stephendonner completed via brave/brave-core#21398 (comment).

[stephendonner]

Verified PASSED using

Brave | 1.62.149 Chromium: 121.0.6167.75 (Official Build) (x86_64)
-- | --
Revision | ff84587bd70af9fcbcbe59fc5194ca65082759c4
OS | macOS Version 11.7.10 (Build 20G1427)

Steps:

1. installed 1.62.149
2. launched Brave using --env-leo=staging --env-ai-chat.bsg=dev --env-ai-chat-premium.bsg=dev --enable-logging=stderr
3. clicked on Leo in the sidebar
4. typed test
5. clicked on Accept and begin
6. loaded wired.com
7. loaded an article
8. clicked on Summarize this page
9. clicked on Suggest questions...
10. opened brave://settings/leo-assistant

Confirmed Mixtral as the default non-Premium LLM

example	example	example	example

1st launch	2nd launch	default settings