Use json encoding for vcs-repo request params when creating a Stack

[brandonc]

Description

Use json encoding for vcs-repo request params when creating a Stack

Testing plan

This problem became apparent when using github-app-installation-id parmeter

stack1, err := client.Stacks.Create(ctx, StackCreateOptions{
    Name: "mm-test-stack",
    VCSRepo: &StackVCSRepo{
        Identifier:        "hashicorp/pet-nulls-stack",
        GHAInstallationID: "ghain-waqUDydU45bDADCe",
    },
    Project: &Project{
        ID: orgTest.DefaultProject.ID,
    },
})

[Uk1288]

LGTM 🎉

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.