Revamping EC3 Widget's Template Creation Workflow

[parideshpande]

As the EC3 widget is being integrated in itwin-experience to surface carbon calculation, there is a UX revamp required for the widget. I have broken down the revamp process into parts, and this PR covers the 1st part only. Considering that this is part 1, following screens would be included in the scope of this PR-

• Change the landing page of no templates available to the one mentioned in the mockup
• Change the template creation workflows and combine the template and assembly creation process
• Add support for multiple assembly creation
• Support Edit facility for 2, 3
• Keep the export functionality as it is. Replace the 'Export' Button in the mock-up with 'Save'

Out of scope -

• Starting export from the create step
• Revamping screen for template list

Note: I have deleted all the unused files from the previous implementation, in order to keep the code clean

[CLAassistant]

All committers have signed the CLA.

[Guillar1]

Eventually Template.ts has to be removed and replaced with types from @itwin/insights-client. Configuration --> EC3Configuration