feat: cache token for GCloudAuthorizedUser

[liufuyang]

Current implementation for GCloudAuthorizedUser is not caching the token got by gcloud auth print-access-token. The token is valid for 60m but when using gcp_auth to run some service locally the gcloud command is run for each auth_manager.get_token(...) call.

This PR adds a token: RwLock<Token> in GCloudAuthorizedUser just like other accounts that implements ServiceAccount, also making the use cases more efficient when calling with auth_manager.get_token() by using GCloudAuthorizedUser.

---

Demo - effects when updated:
Before (gcloud auth print-access-token is run every time when using the manager get_token(...)):

fuyangl@Fuyangs-MacBook-Pro:~/workspace/bigtable_rs 
(main $) 🦀 👉  curlt localhost:3030/key1
    time_namelookup:  0.006677
       time_connect:  0.007068
    time_appconnect:  0.000000
   time_pretransfer:  0.007101
      time_redirect:  0.000000
 time_starttransfer:  0.800651
                    ----------
         time_total:  0.801008
fuyangl@Fuyangs-MacBook-Pro:~/workspace/bigtable_rs 
(main $) 🦀 👉  curlt localhost:3030/key1
    time_namelookup:  0.006588
       time_connect:  0.007086
    time_appconnect:  0.000000
   time_pretransfer:  0.007133
      time_redirect:  0.000000
 time_starttransfer:  0.688113
                    ----------
         time_total:  0.688453

After (token is cached):

fuyangl@Fuyangs-MacBook-Pro:~/workspace/bigtable_rs 
(main *) 🦀 👉  curlt localhost:3030/key1
    time_namelookup:  0.006734
       time_connect:  0.007171
    time_appconnect:  0.000000
   time_pretransfer:  0.007209
      time_redirect:  0.000000
 time_starttransfer:  0.047468
                    ----------
         time_total:  0.047594
fuyangl@Fuyangs-MacBook-Pro:~/workspace/bigtable_rs 
(main *) 🦀 👉  curlt localhost:3030/key1
    time_namelookup:  0.006815
       time_connect:  0.007263
    time_appconnect:  0.000000
   time_pretransfer:  0.007308
      time_redirect:  0.000000
 time_starttransfer:  0.049509
                    ----------
         time_total:  0.049676

[liufuyang]

@hrvolapeter Please take a look? 😃

[liufuyang]

Code updated.

[djc]

Nice! One small nit.

[hrvolapeter]

This have been released in 0.7.4

[liufuyang]

Thank you, that is nice to know :)