feat: staging function to extract element text into one string (#1741)
### Summary
In order to enable larger scale testing of the new text extraction
metrics, create a helper function to get the clean, concatenated text
(CCT) from partitioned elements.
### Test
Partition any file, then pass the resulting elements into the new
`elements_to_text` function. Can test getting the output as string or as
text file.
```
from unstructured.partition.auto import partition
from unstructured.staging.base import elements_to_text
elements = partition(filename="example-docs/chevron-page.pdf", strategy="hi_res")
elements_text = elements_to_text(elements, "output-text-file.txt")
print(elements_text)
```