feat: use token from local fs path

[abeatrix]

Context: https://sourcegraph.slack.com/archives/C05AGQYD528/p1690833224615029?thread_ts=1690216157.663499&channel=C05AGQYD528&message_ts=1690833224.615029

For users that do not have keychain available on their local machine, they can define the absolute path of the file that has the token available in JSON format.

Step 1: Sign into Cody with a valid token

Step 2: Create a JSON file to store the token you used to log in from Step 1 using the following format, save the file as token.json:

{
  "token": "token_string"
}

Step 3: in Command Palette, select Open User Settings (JSON) and add the following field:

{
  "cody.experimental.localTokenPath": "/Users/username/token.json"
}

Step 4: Replace /Users/username/token.json with the file path created from Step 2

Step 5: Reload VS Code, you should still be signed into VS Code

NOTE: This is set behind the experimental flag and remains undocumented as this should only be used as workaround at users' own risk.

Test plan

Follow the steps as described above

[philipp-spiess]

LGTM