Centrally define the A2UI extension URI in typescript code

[jacobsimionato]

Currently, there are 6 places where we refer to the string https://a2ui.org/ext/a2a-ui/v0.8 after https://github.com/google/A2UI/pull/209

We should have one place, so that it doesn't get out of sync, and in case we change our mind about what that string should be.

There should be some central place we refer to the extension name. There might also be an opportunity to share a bit more related logic around extension advertisement.

[jacobsimionato]

CC @ava-cassiopeia as a refactor idea

[ava-cassiopeia]

Always here for more refactors 👀