[Java]: Easier support for modifying scopes

### Clear and concise description of the problem

Java generates the following

```
@useAuth(
ApiKeyAuth | OAuth2Auth<[
{
type: OAuth2FlowType.implicit,
authorizationUrl: "https://login.microsoftonline.com/common/oauth2/v2.0/authorize",
scopes: ["https://search.azure.com/.default"],
}
]>
)
```

with `scopes` being a `private static final String[]` with the scope value in the definition as the only value. For example:

```java
@Generated
private static final String[] DEFAULT_SCOPES = new String[] { "https://search.azure.com/.default" };
```

and later when creating a client

```java
if (tokenCredential != null) {
policies.add(new BearerTokenAuthenticationPolicy(tokenCredential, DEFAULT_SCOPES));
}
```

For services that require sovereign cloud audience scope support this means writing complicated post-code generation customizations.

Instead, to simplify this, Java could continue to generate `DEFAULT_SCOPES` but add in a `private String[] scopes` that defaults to `DEFAULT_SCOPES` that can be mutated but does expose a method to do so. And if a service needs sovereign cloud support it could add a non-`@Generated` method that will be retained if `partial-update` is enabled. For example,

```java
@Generated
private static final String[] DEFAULT_SCOPES = new String[] { "https://search.azure.com/.default" };

@Generated
private String[] scopes = DEFAULT_SCOPES;
```

and in client creation

```java
if (tokenCredential != null) {
String[] scopeToUse = (scopes == null) ? DEFAULT_SCOPES : scopes;
policies.add(new BearerTokenAuthenticationPolicy(tokenCredential, scopes));
}
```

and if a service needs to add sovereign cloud support it could be as easy as

```java
/**
* Sets the Audience to use for authentication with Microsoft Entra ID.
*

* The audience is not considered when using a {@link #credential(AzureKeyCredential) shared key}.
*

* If {@code audience} is null the public cloud audience will be assumed.
*
* @param audience The Audience to use for authentication with Microsoft Entra ID.
* @return The updated SearchClientBuilder object.
*/
public SearchClientBuilder audience(SearchAudience audience) {
if (audience == null) {
this.scopes = DEFAULT_SCOPES;
} else {
this.scopes = new String[] { audience.toString() }; // the string from SearchAudience is the appropriate value for the cloud target
}
return this;
}
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.