Create KeycloakRealmResource to represent the address of a realm

## Background and Motivation

A Keycloak server may have several realms and the realms are what's important for applications.

But, at the moment, `JwtBearerOptions` does not support service discovery for authority and Keycloak has its own way of constructing the base address of the realm.

It would be helpful, if "creating" a realm would create connection string for the realm.

## Proposed API

On the Aspire host, there would be a `AddReal` method that would create a connections string to the realm.

```csharp
var keycloak = builder.AddKeycloak(...);
var realm = keycloak.AddRealm(...);

var service = builder.AddProject<...>(...)
.WithEnvironment("Security__Authentication__JwtBearerOptions__Authority", realm)
:
```

## Usage Examples

On the service side, the service would only have to register the configuration:

```csharp
builder.Services
.AddOptions("Bearer")
.BindConfiguration("VisionBox:Security:Authentication:JwtBearerOptions")
.ValidateOnStart();
-->

## Complementary Designs

Import realm could return a connection string for the realm.

Start by reviewing the existing Keycloak resource and import APIs, then trace how JwtBearerOptions currently receives its authority configuration. Define what the proposed AddRealm API and realm connection string must represent, and consider the usage examples as the completion criteria.

Written by the indexing model from the issue text.