Extension Framwork: Add Support for `init` event

[wbreza]

Today the extension framework supports hooking into project and service level events including restore, build, package, provision & deploy

init is missing from this list since azd does not currently support init hooks. Adding init hooks & event support would enable interesting scenarios where extension developers would have the opportunity to perform interesting extensions as a azd template/project is being initialized.

Some examples of this would be some of the SJAD work that has some customizations to easy init for java projects. After implementing these init hooks I would be an easy task to transition this work out of the azd core and into a java extension for azd that can handle these specific java scenarios.

[spboyer]

Closing as duplicate of #5683 which covers the same scope and is assigned to @JeffreyCA.