Declare PathTemplate is Immutable

[iamdanfox]

When hand-rolling a dialogue client, I got these annoying warnings:

> Task :multipass-oauth2-client:compileJava
/Volumes/git/multipass-grant-types/multipass-oauth2-client/src/main/java/com/palantir/multipass/client/api/MultipassOAuth2Endpoints.java:16: warning: [ImmutableEnumChecker] enums should be immutable: 'token' has field 'pathTemplate' of type 'com.palantir.dialogue.PathTemplate', the declaration of type 'com.palantir.dialogue.PathTemplate' is not annotated with @com.google.errorprone.annotations.Immutable
        private final PathTemplate pathTemplate =
                                   ^
    (see https://errorprone.info/bugpattern/ImmutableEnumChecker)
/Volumes/git/multipass-grant-types/multipass-oauth2-client/src/main/java/com/palantir/multipass/client/api/MultipassOAuth2Endpoints.java:46: warning: [ImmutableEnumChecker] enums should be immutable: 'revokeToken' has field 'pathTemplate' of type 'com.palantir.dialogue.PathTemplate', the declaration of type 'com.palantir.dialogue.PathTemplate' is not annotated with @com.google.errorprone.annotations.Immutable
        private final PathTemplate pathTemplate =
                                   ^
    (see https://errorprone.info/bugpattern/ImmutableEnumChecker)
2 warnings

[svc-autorelease]

Released 1.27.0