Improve si-select customization

[spliffone]

Prerequisites

• I have read the Contributing Guidelines.
• I have not leaked any internal/restricted information like screenshots, videos, code snippets, links etc.

Feature request

Currently the si-select has the following limitations regarding the styling:

• it should be possible to change the select to a button appearance e.g. by applying the classes .btn .btn-secondary
• it should be possible to adjust the block size for drop down items when using the siSelectOptionTemplate which allows to display additional information in a second row of the item
• it should be possible to provide a template for the appearance in the si-select-input other than the siSelectOptionTemplate

Use case

No response