feat(oauth): Use keyring to store oauth token

[burmudar]

This adds a keyring which is used to store OAuth credentials. The OAuth credentials are retrieved from the keyring whenever there is no SRC_ACCESS_TOKEN defined.

When we use an OAuth token, we also add a OAuth transport which will automatically refresh the access token using the refresh token when it is near to expiring.

Credentials are stored in the keyring with under the service "Sourcegraph CLI" and with key "src:oauth:". The reason for the endpoint being in the key name is that some clients might access multiple sourcegraph instances like dev and prod, and we can't reuse the same credentials for the different servers.

Test plan

• Authorize by running src login --oauth https://sourcegraph.sourcegraph.com
• run export SRC_ENDPOINT=https://sourcegraph.sourcegraph.com; echo 'query { currentUser { username } }' | go run ./cmd/src api
• Revoke access of Sourcegraph CLI application and rerun previous command

The OAuth token is invalid. Please check that the Sourcegraph CLI client is still authorized.

To re-authorize, run: src login

Learn more at https://github.com/sourcegraph/src-cli#readme

error: 401 Unauthorized

Invalid OAuth token.
exit status 1

Platforms tested

• Mac
• Windows
• Linux

[burmudar]

This change is part of the following stack:

• feat(oauth): use oauth device flow to authenticate with predefined src-cli OAuth client  #1223
  • feat(oauth): Add refresh to oauthdevice.Client #1227
    • feat(oauth): Use keyring to store oauth token #1228 ◀

_{Change managed by git-spice.}

[burmudar]

opted to not store anything when failing to store in the keyring. This means if we fail storing the user would have to login everytime - not exactly ideal.

We could write the details to disk and warn the user? And everytime we load it we warn them again?