We would like to revisit the `Authentication` class to make it provide better and easier to use interface to the consumers. 

#79809 enables run-as for all authentication schemes in addition to `realm` which is already supported. This adds more complexity to an `Authentication` object and how it should be used. For an example, building the role associated to an user now has to consider whether the user is the authenticated user or run-as user and the authentication scheme of the authenticated user. The existing `Authentication` class can answer all these question today. But its interface and internals are not aligned for these questions and the usage is error prone. For example, the `Authentication` object itself does not know whether the user has run-as. This information is kept by the `User` object. If the `User` object has an `authenticatedUser`, it's the run-as user. Otherwise it is not. But the information about realms for each `User` is directly kept in `Authentication`. Therefore, for full picture, the caller must check both `User` and `Authentication` realms and understands the nuance of their differenent combinations. 

- [x] Refactor CompositeRolesStore to separate roleDescriptors resolving (#80926)
- [ ] Move effective role descriptor checking from Resolver to CompositeRolesStore
- [x] https://github.com/elastic/elasticsearch/issues/81192
- [ ] Replace Authentication with AuthenticationContext in places other than CompositeRolesStore
x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/security/authc/AuthenticationSerializationTests.java
public void testWriteToAndReadFrom() throws Exception {
        User user = new User(randomAlphaOfLengthBetween(4, 30), generateRandomStringArray(20, 30, false));
        BytesStreamOutput output = new BytesStreamOutput();
        AuthenticationSerializationHelper.writeUserTo(user, output);
        User readFrom = AuthenticationSerializationHelper.readUserFrom(output.bytes().streamInput());
        assertThat(readFrom, not(sameInstance(user)));
        assertThat(readFrom.principal(), is(user.principal()));
        assertThat(Arrays.equals(readFrom.roles(), user.roles()), is(true));
        assertThat(readFrom, not(instanceOf(Authentication.RunAsUser.class)));
    }
    public void testWriteToAndReadFromWithRunAs() throws Exception {
        User authUser = new User(randomAlphaOfLengthBetween(4, 30), generateRandomStringArray(20, 30, false));
        User user = new Authentication.RunAsUser(
            new User(randomAlphaOfLengthBetween(4, 30), randomBoolean() ? generateRandomStringArray(20, 30, false) : null),
            authUser
        );
        BytesStreamOutput output = new BytesStreamOutput();
        AuthenticationSerializationHelper.writeUserTo(user, output);
        User readFrom = AuthenticationSerializationHelper.readUserFrom(output.bytes().streamInput());
        assertThat(readFrom, not(sameInstance(user)));