Service Discovery: allow multiple schemes to be specified in request

[ReubenBond]

This PR adds support in Service Discovery for specifying multiple schemes during resolution, separated by a + character. This allows developers to enforce HTTPS in production while falling back to HTTP during development and testing.
For example, a user can allow both HTTPS and HTTP like so:

builder.Services.AddHttpClient<CatalogServiceClient>(
  c => c.BaseAddress = new("https+http://catalogservice"));

The order of schemes is important: if endpoints are found matching the first scheme, no endpoints from subsequent schemes are allowed.

This PR also introduces configuration to specify an allow-list of acceptable schemes for service discovery:

For example, to allow only https://:

builder.Services.Configure<ServiceDiscoveryOptions>(options =>
{
    options.AllowedSchemes = ["https"];
});

To allow http:// or https://:

builder.Services.Configure<ServiceDiscoveryOptions>(options =>
{
    options.AllowedSchemes = ["https", "http"];
});

To allow any scheme (the default):

builder.Services.Configure<ServiceDiscoveryOptions>(options =>
{
    options.AllowedSchemes = ServiceDiscoveryOptions.AllSchemes;
});

This PR changes the format of Service Discovery configuration so that the endpoint name is included in the configuration path, if an endpoint name is set. If no endpoint name is specified, then the scheme is used (and again, schemes are checked in the specified order, with only the first present being selected). If no endpoint name or scheme is specified, "default" is used as the endpoint name.

Microsoft Reviewers: Open in CodeFlow

[davidfowl]

clean