Add summary for tile attribute to sentinel-1-gobal-coherence collection metadata

[asjohnston-asf]

tile is an attribute users may find useful to search on, so we should advertise it in the collection summaries at https://github.com/ASFHyP3/asf-stac/blob/develop/collections/sentinel-1-global-coherence/sentinel-1-global-coherence.json#L32

It's impracticable to list all possible values, so we'll likely provide a jsonschema with a regular expression and an example per https://github.com/radiantearth/stac-spec/blob/master/collection-spec/collection-spec.md#summaries

"tile": {
    "type": "string",
    "pattern": "....",
    "example": "N01W001"
}