Always make the baseUrl overridable in clients

[timotheeguerin]

Tcgc issue from the discusssion here #217

As discussed the baseUrl (https://foo.com/bar) should always be settable in clients as an option.

Issue was also filed to see if http library should also provide a different view microsoft/typespec#3821 in the future

Just to clarify here too but it should be the baseUrl that is overridable not the host(Basically the entire thing that @server would set. Without that you are blocking many scenarios still:

• http:// localhost
• different hosted path

[timotheeguerin]

Playground showcasing the issues

import "@typespec/http";

using TypeSpec.Http;

// ---------------------- How server should work in theory ----------------------
@service
@server("https://github.com", "Github public api")
namespace Service1 {
// new Service1Client();
// new Service1Client({baseUrl: "http://localhost:3000"});

}

@service
@server(
  "https://{region}.azure.com",
  "Region service",
  {
    region: string,
  }
)
namespace Service2 {
// new Service2Client({region: "westus"});
// new Service2Client({baseUrl: "http://localhost:3000"});

}

@service
@server("https://github.com", "Github public api")
@server(
  "https://{org}.private.github.com",
  "Github private org api",
  {
    org: string,
  }
)
@server(
  "{endpoint}",
  "hosted github api",
  {
    endpoint: url,
  }
)
namespace Service3 {
// Service3Client.publicApi()
// Service3Client.privateApi("my-org")
// Service3Client.hosted("https://my-intra-custom-github.com")
// new Service3Client({baseUrl: "http://localhost:3000"});

}

// ----------------------     Azure  cases ----------------------
@service
@server("https://management.azure.com", "Azure public API")
namespace ArmServiceOption1 {
// new ArmServiceOption1Client();
// new ArmServiceOption1Client({baseUrl: "https://fairfax.azure.com"});

// Pros:
// - simple
// Cons:
// - not actually accurate, tcgc could special case to emit similar as option3

}
@service
@server("https://management.azure.com", "Azure public API")
@server("https://fairfax.azure.com", "Fairfax public API")
@server("https://china.azure.com", "China public API")
namespace ArmServiceOption2 {
// ArmServiceOption1Client.public();
// ArmServiceOption1Client.fairfax();
// ArmServiceOption1Client.china();

// Pros:
// - accurate
// Cons:
// - expose (in theory) all the gov cloud as typed factory which is not ideal
// - worse for public cloud

}

@service
@server(
  "{azureEndpoint}",
  "Arm endpoint",
  {
    azureEndpoint: url = "https://management.azure.com",
  }
)
namespace ArmServiceOption3 {
// new ArmServiceOption3Client();
// new ArmServiceOption3Client({endpoint: "https://fairfax.azure.com"});
// new ArmServiceOption3Client({baseUrl: "https://fairfax.azure.com"});

// Pros:
// - accurate
// Cons:
// - autorest output will change or autorest library will need to revert back

}