feat(cli): Dynamic snippets API support custom base URLs

[amckinney]

This adds the ability for dynamic snippet callers to customize the environment and/or base URL used in the generated snippet. There are a few use cases to consider:

1. Environment by ID

This works for both single environment and multi environment SDKs.

const response = await go.endpoint("POST /users").generate({
  environment: "Staging",
  requestBody: { ... }
})

client := client.NewClient(
    option.WithBaseURL(
       acme.Staging // Reference the generated environment enum.
    ),
)

2. Single environment with custom base URL

If both baseURL and environment are set, we return a validation error.

const response = await go.endpoint("POST /users").generate({
  baseURL: "http://localhost:8080"
  requestBody: { ... }
})

client := client.NewClient(
    option.WithBaseURL(
       "http://localhost:8080" // Use the raw url string.
    ),
)

3. Multiple environment with custom URL

Expects an object with a multiple keys, with each environment ID as a key w/ a base URL value.

Note that the WithEnvironment option isn't actually supported in the Go SDK yet, but this at least
outlines the expected use case (which is applicable to other languages, too).

const response = await go.endpoint("POST /users").generate({
  environment: {
    ec2: "http://localhost:8080",
    aws: "http://localhost:8081"
  },
  requestBody: { ... }
})

client := client.NewClient(
    option.WithEnvironment(
       acme.Environment{
         Ec2: "http://localhost:8080",
         Aws: "http://localhost:8081"
       }
    ),
)