Expose Maya Attribute to USD attribute conversion to maya usd lib python API

**Is your feature request related to a problem? Please describe.**

It'd be great if we can expose the [`UsdMayaWriteUtil::GetOrCreateUsdAttr`](https://github.com/Autodesk/maya-usd/blob/9ae146caea140154ca536eea6e4b8ea4715fe889/lib/mayaUsd/fileio/utils/writeUtil.cpp#L151) logic to the `mayaUsd.lib` python API or one of the lower level methods like just the type conversion as [here](https://github.com/Autodesk/maya-usd/blob/9ae146caea140154ca536eea6e4b8ea4715fe889/lib/mayaUsd/utils/converter.cpp#L1160).

By doing so it might make it easier to write custom Python Export chasers to write out certain attributes.

Imagine a chaser (like the "alembic" chaser) that mimics the `-attr` and `-attrPrefix` flags Maya's Alembic exports had.
Here's a very quick and dirty boilerplate:
```python
import mayaUsd.lib as mayaUsdLib
from pxr import Usd
import maya.api.OpenMaya as om

def get_mplug(plug):
sel = om.MSelectionList()
sel.add(plug)
return sel.getPlug(0)

class CustomAttributesChaser(mayaUsdLib.ExportChaser):

def __init__(self, factoryContext, *args, **kwargs):
super(CustomAttributesChaser, self).__init__(factoryContext, *args, **kwargs)

# TODO: Actually parse the attribute names from the chaser args
self._attrs = ["my_attribute", "my_prefix*"]

self._stage = factoryContext.GetStage()
self._dag_to_usd = {str(item.key()): item.data() for item in factoryContext.GetDagToUsdMap()}

# def ExportDefault(self):
# return self.ExportFrame(Usd.TimeCode.Default())

# def ExportFrame(self, frame):
# return True

def PostExport(self):

stage = self._stage
for maya_node, usd_path in self._dag_to_usd.items():
attrs = cmds.listAttr(maya_node, st=self._attrs)
if not attrs:
continue

prim = stage.GetPrimAtPath(usd_path)
for attr in attrs:
plug = get_mplug(f"{maya_node}.{attr}")

# TODO: Expose this
#mayaUsdLib.GetOrCreateUsdAttr(
# attrPlug=plug,
# usdPrim=prim,
# attrName=attr,
# custom=False,
# translateMayaDoubleToUsdSinglePrecision=True
#)

# And then actually setting the values..

return True

# Unregister and register again for easy development
mayaUsdLib.ExportChaser.Unregister(CustomAttributesChaser, "attrib")
mayaUsdLib.ExportChaser.Register(CustomAttributesChaser, "attrib")

# Perform a test export with the chaser
from maya import cmds
cmds.loadPlugin("mayaUsdPlugin", quiet=True)
cmds.mayaUSDExport(file="E:/test.usd",
frameRange=(1, 5),
frameStride=1.0,
chaser=["attrib"],
selection=True)
```

**Describe the solution you'd like**

Would be great if there was a trivial way to write a matching attribute (and maybe even write samples for a given frame, or multiple frames?) from that Maya attribute to the USD attribute in an easy way.

**Describe alternatives you've considered**

Writing and maintaining the (quite complex?) code myself..

Start by reading the referenced logic in lib/mayaUsd/fileio/utils/writeUtil.cpp and the type-conversion code in lib/mayaUsd/utils/converter.cpp. Check how the mayaUsd.lib Python API exposes related functionality and how ExportChaser is used. Done means a custom Python export chaser can convert and write Maya attributes to USD attributes, including values for export frames.

Written by the indexing model from the issue text.