Enable Expression to be created without an operator

[pjleonard37]

Resolves #1571

This PR updates Expression, adding an init that does not require an operator. With this change, GeoJSONSource's clusterProperties will align with the style spec (see here). Following the style spec, a cluster property needs to be an array of two elements with several accepted formats:

["propertyKey" : [operator, [mapExpression]]] 
["propertyKey" : [[operator, ['accumulated'], ['get', propertyKey]], [mapExpression]]] 
["propertyKey" : [[reduceExpression], [mapExpression]]]

Currently, only the first format works, and not more complex options described in the style spec:

For more advanced use cases, in place of operator, you can use a custom reduce expression that references a special ["accumulated"] value, e.g.: {"sum": [["+", ["accumulated"], ["get", "sum"]], ["get", "scalerank"]]}

This PR allows for Expression creation without an operator so that the additional formats work. I additionally updated the Symbol Clustering Example to show how the clusterProperties API can work with these formats.

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[maios]

can we also support decoding an operatorless Expression? Right now Expression decode requires the first element to be an Operator

[pjleonard37]

I removed this helper function as it was only used once

[pjleonard37]

Thanks for the reviews @maios & @zmiao! I've updated the code based on your comments.

[codecov]

Codecov Report

Merging #1855 (621e778) into main (4b54016) will increase coverage by 0.01%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##             main    #1855      +/-   ##
==========================================
+ Coverage   89.06%   89.08%   +0.01%     
==========================================
  Files         242      242              
  Lines       15080    15094      +14     
==========================================
+ Hits        13431    13446      +15     
+ Misses       1649     1648       -1     

Flag	Coverage Δ
integration	56.67% <62.96%> (-0.02%)	⬇️
unit	83.62% <92.59%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ources/MapboxMaps/Annotations/ClusterOptions.swift	100.00% <ø> (ø)
Sources/MapboxMaps/Style/Types/Expression.swift	81.76% <92.59%> (+0.33%)	⬆️
...boxMaps/Annotations/OffsetGeometryCalculator.swift	67.97% <0.00%> (-0.57%)	⬇️
Sources/MapboxMaps/Offline/OfflineErrors.swift	47.36% <0.00%> (+3.50%)	⬆️
Sources/MapboxMaps/Style/SourceType.swift	93.75% <0.00%> (+12.50%)	⬆️