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Similar to matcher::narrowPath (SERVER-134593, SERVER-135339, SERVER-134936), but for aggregation pipelines!

It should accept the PathSemantics defined in SERVER-135978.

Type narrowPath(Type inputType, FieldPath path, PathSemantics semantics,  Type constraint, bool assumeTrue);

And will produce a Type derived from inputType which is narrowed by the constraint type.

We will only implement PathSemantics::kFieldPath, but we should support dotted paths!

Examples:

{a: any, b: any}
with path 'a' with constraint number produces {a: number, b: any}

{a: any, b: any}
with path 'a.b' with constraint number produces {a:
{b: number}
, b: any}

- This is similar to {$expr: {$isNumber: '$a.b'}} for example which implies 'a' is non-array since the result is number.
- Or {$expr: {$eq: ['$a.b', 1]}} which also implies non-array, but the type returned here should be {a: {b: number(S)}} (only subset of numbers).

depends on

SERVER-135978 Implement expression::traversePath

is related to

SERVER-134936 Implement matcher::narrowPath for dotted paths

SERVER-135339 Implement matcher narrowing with LeafArrayBehavior::kNoTraverse

SERVER-134593 Add basic matcher path traversal and narrowing