Support .svg as Icon for Add-Ins

Currently, the svg Icons, when used as both normal and high resolution Icons for O365 Add-Ins, work in browser. However, the validation infra does not validate the svg icons in the manifest.xml.

npx -y office-addin-manifest validate manifest.xml

It produces the following error:

Error #1:
Unsupported High Resolution Icon URL File Extension: The manifest high resolution icon URL must have a valid image file extension. Please see https://aka.ms/oackhighresolutionicon for details.
- Details: svg

Error #2:
Unsupported Icon URL File Extension: The manifest icon URL must have a valid image file extension. Please see https://aka.ms/oackicon for details.
- Details: svg

The manifest is not valid.

Please consider supporting the svg icons officially, so that we, the Add-In developers, don't need to maintain the icons of different dimensions.

Additional notes:

- The links in the error messages do not seem to redirect to proper Information.

- As far as I have understood it, the actual validation is happening remotely, as can be seen here:

https://github.com/OfficeDev/Office-Addin-Scripts/blob/a686eb3727af027dfa1f4ef3cb8ab3e0df3f821a/packages/office-addin-manifest/src/validate.ts#L99