* Implement Filter: Contains functionality for describe_params
This commit adds the Contains functionality. Tests were created
to mimic behavior in AWS/boto3, including that filters with values
in the form of `/name` will match parameters named `/name/match` but
not parameters named `match/with/other-name`. In the test example, a
Contains filter with the value `/tan` would match: `/tangent-3` and
`tangram-4` but not `standby-5`.
* Enforce parameter filter restrictions on get_parameters_by_path
According to the boto3 documentation [1], `Name`, `Path`, and `Tier` are not
allowed values for `Key` in a parameter filter for `get_parameters_by_path`.
This commit enforces this by calling `_validate_parameter_filters` from the
`get_parameters_by_path` method, and adding a check to `_validate_parameter_filters`.
I added 3 test cases to `test_get_parameters_by_path` which check for the correct
exception when calling with a parameter filter using any of these keys.
* Code formatted to match style
* Refactored logic
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