CLOUDP-330236: Adds NewServiceAccountTransport

[cveticm]

Proposed changes

Adds new transport for Service Accounts.

Unit testing:

Added two new unit tests for transport.go

• TestNewAccessTokenTransport
• TestNewServiceAccountTransport

Manual Testing:

I have tested this transport type by implementing the transport into the client creator logic (This implementation will be put for review as a part of CLOUDP-329787) and manually setting up SA credentials in the config. I've also ensured that the transport method continues to work even after the first token generated by this transport type expired to ensure refresh logic is working.

truncated config file:

version = 2

[oauth2]
client_id = 'mdb_sa_id_<...>'
client_secret = 'mdb_sa_sk_<...>'
auth_type = 'service_account'
org_id = '670cd17af33cea212ea1ed61'
project_id = '670e34d35a4f587387db2102'
service = 'cloud'

Cmd calls using Service Accounts profile

Jira ticket: CLOUDP-330236

Checklist

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation in document requirements section listed in CONTRIBUTING.md (if appropriate)
• I have addressed the @mongodb/docs-cloud-team comments (if appropriate)
• I have updated test/README.md (if an e2e test has been added)
• I have run make fmt and formatted my code

Further comments

[github-actions]

APIx Bot : a message has been sent to Docs Slack channel 🚀.

[fmenezes]

what if is "" ?

[cveticm]

clientcredentials.NewConfig already sets TokenURL and RevokeURL but points them to https://cloud.mongodb.com.

So this logic is to ensure we point to the correct cloud environment if OpsManagerURL is pointing to dev or qa.

[github-actions]

Coverage Report 📈

Branch	Commit	Coverage
SA_refactor_feature_branch	f9dd12d	26.0%
CLOUDP-330236_SA_transport	58b5eb9	26.0%
	Difference	0%

[coveralls]

coverage: 57.846% (-0.1%) from 57.953%
when pulling 7cc5cb0 on CLOUDP-330236_SA_transport
into b0af9fc on master.