Helm Chart - Add configuration for node selector / tolerations / affi…

For some experimentation I'm doing, I need to pin Dendrite to a specific node pool. This should be available configuration within the chart ideally, and this should do it in theory. ### Pull Request Checklist <!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request --> * [x] I have added Go unit tests or [Complement integration tests](https://github.com/matrix-org/complement) for this PR _or_ I have justified why this PR doesn't need tests * [x] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately Signed-off-by: `Rhea Danzey <rdanzey@element.io>`