sdk/identity build broken due to oauth2 package update

[karataliu]

error[E0277]: `?` couldn't convert the error to `RequestTokenError<oauth2::reqwest::Error<reqwest::Error>, StandardErrorResponse<BasicErrorResponseType>>`
  --> sdk\identity\src\authorization_code_flow.rs:94:19

The package worked with 4.0.0-alpha.2 but not 4.0.0-alpha.3. Should lock to 4.0.0-alpha.2 in code since alpha can have breaking changes

[rylev]

This was fixed by #136