add `weaver.wps_client_headers_filter` setting to filter WPS provider headers

[fmigneault]

Changes

• Add weaver.wps_client_headers_filter setting that allows filtering of specific WPS request headers from the incoming request to be passed down to the WPS client employed to interact with the WPS provider.

References

• fixes #600

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (fcec447) 85.64% compared to head (d234e11) 85.65%.

Files	Patch %	Lines
weaver/wps/utils.py	87.50%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #603 +/- ## ======================================= Coverage 85.64% 85.65% ======================================= Files 79 79 Lines 18543 18559 +16 Branches 2838 2844 +6 ======================================= + Hits 15882 15896 +14 - Misses 1925 1926 +1 - Partials 736 737 +1 ```

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