Add Admin Settings API

[omarismail]

Description

This PR adds all the Admin Settings API.

It implements the API for each Settings:

• General.Read
• General.Update
• CostEstimation.Read
• CostEstimation.Update
• SAML.Read
• SAML.Update
• SAML.RevokeIdpCert
• SMTP.Read
• SMTP.Update
• Twilio.Read
• Twilio.Update
• Twilio.Verify
• Customization.Read
• Customization.Update

Testing plan

1. Pull down and run the tests

Output from tests

$ TFE_TOKEN=<token> TFE_ADDRESS=<address>  ENABLE_TFE=1 go test -v ./...  -timeout=30m
...
PASS
ok  	github.com/hashicorp/go-tfe	510.513s
?   	github.com/hashicorp/go-tfe/examples/organizations	[no test files]
?   	github.com/hashicorp/go-tfe/examples/workspaces	[no test files]

External links

• API documentation

[petems]

Awesome, thanks @omarismail! 🤝

[chrisarcand]

Lookin' good!

I skimmed a lot and had enough feedback to stop and revisit in more detail a second time around; the biggest bit is to double check all these APIs to see if they're all covered in the structs; I think a lot of the documentation around these APIs is woefully out of date 😩

[chrisarcand]

Two quick nits and this is ready to merge!