TCE-807: Browser login

[bcmdarroch]

🛠️ Description

This PR introduces the ability to log in via the browser when no client credentials are configured on the HCP Go SDK. 🎉

It uses the OAuth2 implicit flow, implemented by golang's standard oauth2 library, to direct users to a browser to login with their username and password. On successful authentication, the Go SDK is configured with an HCP access token that can be used for up to an hour.

Follow-up work includes the creation of an auth cache library to enable background token refreshing.

🧪 Test Locally

1. Set the following environment variables

export HCP_ORGANIZATION_ID=...
export HCP_PROJECT_ID=...

2. Run the test client.

$  go run ./cmd/hcp-sdk-go-client/main.go    
The default web browser has been opened at https://auth.hashicorp.com/oauth2/auth. Please continue the login in the web browser.
Success!
Response: &models.HashicorpCloudNetwork20200907Network{CidrBlock:"172.25.16.0/20", CreatedAt:strfmt.DateTime{wall:0x0, ext:63756371090, loc:(*time.Location)(nil)}, ID:"hvn", Location:(*models.HashicorpCloudLocationLocation)(0xc00012f170), ProviderNetworkData:(*models.HashicorpCloudNetwork20200907NetworkProviderNetworkData)(0xc000128570), State:"STABLE"}

Response: &models.HashicorpCloudConsul20210204Cluster{Config:(*models.HashicorpCloudConsul20210204ClusterConfig)(0xc0000adaa0), ConsulVersion:"v1.9.17", CreatedAt:strfmt.DateTime{wall:0x0, ext:63761292414, loc:(*time.Location)(nil)}, DNSNames:(*models.HashicorpCloudConsul20210204ClusterDNSNames)(0xc0002a7640), ID:"consul-cluster", Location:(*models.HashicorpCloudLocationLocation)(0xc000364ff0), ResourceID:"b4aa30a9-86cc-41c4-81d6-3cd3b3c9fb0d", State:"RUNNING"}

🚢 Release Note

Release note for CHANGELOG:

Enable browser login when client credentials are unavailable

👍 Definition of Done

• If no client credentials are configured, the Go SDK can use a token obtained through browser login.

[itsjaspermilan]

Question: I understand why this was taken out but what if the client only provides one of the two credentials? Will the browser login workflow just kick in anyways or do we assume the user wanted to try logging in with creds?

[bcmdarroch]

That's a good point. I'll see about adding a check for when partial credentials are provided. But anytime both client credentials are missing, then we trigger browser login.

[bcmdarroch]

Update: this is blocked while we take care of some configuration changes to support the new identity provider introduced by #114.

[bcmdarroch]

Configuration complete!