Improve add_stac_layer to support pystac.Item

[giswqs]

Fix #940

import leafmap

url = "https://earth-search.aws.element84.com/v1/"
collection = "sentinel-2-l2a"
time_range = "2020-12-01/2020-12-31"
bbox = [-122.2751, 47.5469, -121.9613, 47.7458]
result = leafmap.stac_search(
    url=url,
    max_items=10,
    collections=[collection],
    bbox=bbox,
    datetime=time_range,
    query={"eo:cloud_cover": {"lt": 10}},
    sortby=[{"field": "properties.eo:cloud_cover", "direction": "asc"}],
)
item = result.get_all_items()[-1]
m = leafmap.Map()
m.add_stac_layer(item, assets=["nir", "red", "green"])
m

[github-actions]

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[mergify]

Thanks for the contribution @giswqs 🤩