fix: make extensions that don't use project templates backwards-compatible

[anchpop]

Description

There was recently an addition of pub project_templates: Option<HashMap<String, ExtensionProjectTemplate>> to the struct ExtensionManifest.

This is somewhat problematic on its own because ExtensionManifest has #[serde(deny_unknown_fields)]. So any extension that has this field in its manifest won’t be compatible with any version of DFX that was released prior to this field being added.

This lack of backwards-compatibility might be desirable for extensions that declare project templates (since old versions of DFX wouldn't support the extension's whole feature set). But extensions that don't declare project templates should probably be backwards compatible.

So let's tell serde to be tolerant to the field being omitted using #[serde(default)], and skip serializing it if an extension manifest doesn't declare project templates. This will ensure that projects who generate their manifests programmatically (like the NNS and SNS extensions) don't accidentally lose backwards compatibility.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly. (not necessary I think)
• I have made corresponding changes to the documentation. (not necessary I think)