[core] Add colorbar/legend support

[naschmitz]

Changes:

• Added private _add_legend and _add_colorbar APIs in core.
  • I'm not comfortable exposing a new public API in core until it has been thoroughly reviewed.
• Fixed legend support in geemap non-core.
  • Colors weren't being passed to the constructor properly.
• Simplified the legend/colorbar creation logic (string tokenization).

Design decisions:

• Reversible decision: We don't keep track of "unowned" legends or colorbars in core.
  • Unowned here meaning it's not attached to any particular layer.
  • Using public core APIs, it's not possible to create an unowned map.

Testing:

• Tested programmatic and manual legend and colorbar creation on both core and non-core.