Modify BlockTemplateController to server Block Templates

[tjcafferkey]

Depends on #4969 and #4970

Description

Similar to whats shown here we need to add some methods to the BlockTemplateController which first checks if the user has Block Templates for Woo (such as single-product.html) in their theme and if not server them the default block template files from woo-blocks-plugin/templates/block-templates by making use of the filter added in #4970

Acceptance Criteria

• Given a user has an appropriate Block Template in their theme (e.g. single-product.html), we will not render them the default one from Woo Blocks
• Given a user does not have a Block Template in their theme for a WooCommerce page. We will serve them the default template from woo-blocks-plugin/templates/block-templates if its available.
• Given a user does not a Block Template in their theme, or one available in woo-blocks-plugin/templates/block-templates then we will fall back to the legacy PHP template