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
assertThat(readFrom.principal(), is(user.principal()));
        assertThat(Arrays.equals(readFrom.roles(), user.roles()), is(true));
        assertThat(readFrom, instanceOf(Authentication.RunAsUser.class));
        User readFromAuthUser = ((Authentication.RunAsUser) readFrom).authenticatingUser;
        assertThat(authUser, is(notNullValue()));
        assertThat(readFromAuthUser.principal(), is(authUser.principal()));
        assertThat(Arrays.equals(readFromAuthUser.roles(), authUser.roles()), is(true));
        assertThat(readFromAuthUser, not(instanceOf(Authentication.RunAsUser.class)));
    }
    public void testSystemUserReadAndWrite() throws Exception {
        BytesStreamOutput output = new BytesStreamOutput();
        AuthenticationSerializationHelper.writeUserTo(SystemUser.INSTANCE, output);
        User readFrom = AuthenticationSerializationHelper.readUserFrom(output.bytes().streamInput());
        assertThat(readFrom, is(sameInstance(SystemUser.INSTANCE)));
        assertThat(readFrom, not(instanceOf(Authentication.RunAsUser.class)));
    }
    public void testXPackUserReadAndWrite() throws Exception {
        BytesStreamOutput output = new BytesStreamOutput();
        AuthenticationSerializationHelper.writeUserTo(XPackUser.INSTANCE, output);
        User readFrom = AuthenticationSerializationHelper.readUserFrom(output.bytes().streamInput());
        assertThat(readFrom, is(sameInstance(XPackUser.INSTANCE)));
        assertThat(readFrom, not(instanceOf(Authentication.RunAsUser.class)));
    }
    public void testAsyncSearchUserReadAndWrite() throws Exception {
        BytesStreamOutput output = new BytesStreamOutput();
        AuthenticationSerializationHelper.writeUserTo(AsyncSearchUser.INSTANCE, output);
        User readFrom = AuthenticationSerializationHelper.readUserFrom(output.bytes().streamInput());
        assertThat(readFrom, is(sameInstance(AsyncSearchUser.INSTANCE)));
        assertThat(readFrom, not(instanceOf(Authentication.RunAsUser.class)));
    }