CLOSE https://github.com/sourcegraph/cody-issues/issues/76

See the linked issue for details.

Objectives:

• Add a new site config value called SmartContext
• SmartContext is default to "enabled"
• Update site config docs for ChatModelMaxTokens to indicate it is not respected if SmartContext is enabled, and that instead, Cody will determine the max tokens to use based on the model name

Changes:

• Add SmartContext field to CompletionsConfig struct
• Implement SmartContext() method in codyLLMConfigurationResolver
• Set default SmartContext value to "enabled" if not provided, and "disabled" to disable the feature
• Update schema and documentation to describe SmartContext feature
• Update documentation of chatModelMaxTokens regarding how it would be overrides when SmartContext is enabled

Test plan

1. Run sg start from this branch to start a Sourcegraph instance locally
2. Visit https://sourcegraph.test:3443/api/console#%7B%22query%22%3A%22%23%20Type%20queries%20here%2C%20with%20completion%2C%20validation%2C%20and%20hovers.%5Cn%23%5Cn%23%20Here's%20an%20example%20query%20to%20get%20you%20started%3A%5Cn%5Cnquery%20CurrentSiteCodyLlmConfiguration%20%7B%5Cn%20%20%20%20site%20%7B%5Cn%20%20%20%20%20%20%20%20codyLLMConfiguration%20%7B%5Cn%20%20%20%20%20%20%20%20%20%20%20%20smartContext%5Cn%20%20%20%20%20%20%20%20%7D%5Cn%20%20%20%20%7D%5Cn%7D%22%2C%22operationName%22%3A%22CurrentSiteCodyLlmConfiguration%22%7D to RUN the graphQL query below that the Cody clients would call
3. If "completions.smartContext" is not configured in your site configurations, it will be defaulted to "enabled":

4. Go to your site configurations and set "completions.smartContext" to "disabled"

5. Run the graphQL query again to verify that it returns "disabled":

graphQL query

query CurrentSiteCodyLlmConfiguration {
    site {
        codyLLMConfiguration {
            smartContext
        }
    }
}