Integrate STAC discovery widget

[abarciauskas-bgse]

Add a widget where the library can be configured with one (or more) STAC libraries to find collections and subsequently items.

User should be able to navigate to a form widget where collections from MAAP STAC catalog are selectable, and once a user has selected a STAC collections, they can search for items in that collection and those items load in a drop down.

Next https://github.com/developmentseed/stac_ipyleaflet/issues/8 - Once users have discovered items from the STAC, they should be able to add an item as a layer to the map

AC:

• [x] user can open a dialog box where they see a list of MAAP collections available from our STAC catalog stac.maap-project.org
• [x] user can select a collection from the list and click a "Search" button
• [x] Once the user clicks the search button, another dropdown of items should be populated

See https://github.com/giswqs/leafmap/pull/340

[abarciauskas-bgse]

Julia Signell also suggested https://stackstac.readthedocs.io/en/latest/examples/show.html

[abarciauskas-bgse]

@jjfrench and I met about this today and I updated the AC to match our conversation

[abarciauskas-bgse]

This is complete