Support attribute on MayaReference prim to toggle -mergeNamespacesOnClash flag

**Is your feature request related to a problem? Please describe.**
The `UsdMayaTranslatorMayaReference::LoadMayaReference` function is hard coded to call `-mergeNamespacesOnClash false`. This means that if you create an `AL_usdmaya_ProxyShape` node in a namespace (eg, `my_asset_1:proxy`), that is proxy for an asset, it is not possible for the rig to load in the same namespace. To avoid an off-by-one error (eg, `my_asset_2:rig`), you have to salt the namespace (eg, `_my_asset_1:rig`) or nest the namespace (eg, `rigs:my_asset_1:rig)`. Either option introduces an undesired complexity in naming conventions.

**Describe the solution you'd like**
The `UsdMayaTranslatorMayaReference::LoadMayaReference` function searches for an attribute on the MayaReference prim (eg, `maya_mergeNamespacesOnClash`), whose value is used to set the `-mergeNamespacesOnClash` flag as `true/false`.

The default is `false` to maintain legacy behavior.

**Additional context**
The reason for loading the proxy asset in a namespace is so that the rig can load in the same namespace. If the asset is named "my_asset_1" in the root namespace, then it is not possible to have the rig use "my_asset_1" as a namespace since namespaces must be globally unique.

Start with UsdMayaTranslatorMayaReference::LoadMayaReference, the entry point named in the issue, and inspect how the MayaReference prim is read before the reference is loaded. Add support for the named attribute while preserving false as the default; done means the attribute controls -mergeNamespacesOnClash without changing legacy behavior.

Written by the indexing model from the issue text.