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/main/java/org/elasticsearch/xpack/core/security/authc/Authentication.java
|| AuthenticationType.TOKEN == getAuthenticationType()) {
            return true;
        }
        return false;
    }
    /**
     * Writes the authentication to the context. There must not be an existing authentication in the context and if there is an
     * {@link IllegalStateException} will be thrown
     */
    public void writeToContext(ThreadContext ctx) throws IOException, IllegalArgumentException {
        new AuthenticationContextSerializer().writeToContext(this, ctx);
    }
    public String encode() throws IOException {
        BytesStreamOutput output = new BytesStreamOutput();
        output.setVersion(version);
        Version.writeVersion(version, output);
        writeTo(output);
        return Base64.getEncoder().encodeToString(BytesReference.toBytes(output.bytes()));
    }
    public void writeTo(StreamOutput out) throws IOException {
        AuthenticationSerializationHelper.writeUserTo(user, out);
        authenticatedBy.writeTo(out);
        if (lookedUpBy != null) {
            out.writeBoolean(true);
            lookedUpBy.writeTo(out);
        } else {
            out.writeBoolean(false);
        }
        out.writeVInt(type.ordinal());
        out.writeGenericMap(metadata);