feat(domain discovery): add support for Domain Discovery commands

[FarhanSajid1]

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This PR adds commands for the domain tools. Once a user has enabled Domain Discovery on their accounts, they should have access to the endpoints that enable this.

The commands make use of the status and suggest implementation which were added in fastly/go-fastly#672.

The core changes:

• Add a root command for tools, which can be reused
• Add suggest and status subcommands under tools
• Tests for suggest and status subcommands

Changes to Core Features:

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User Impact

• What is the user impact of this change?

Users that have enabled domain discovery on their accounts can use the suggest and status commands.

Are there any considerations that need to be addressed for release?

No breaking changes

[Integralist]

This looks great! Thanks @FarhanSajid1

Just a couple of minor/nit comments and suggestions (approving to unblock).