Deprecate custom_parse in favor of custom_func

[jf-tech]

Deprecated but still functional. Simply removed mention of custom_parse from doc and marked it Deprecated in public APIs.

#134

[codecov]

Codecov Report

Merging #135 (a779c38) into master (4c56223) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #135   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           43        43           
  Lines         1961      1961           
=========================================
  Hits          1961      1961           

Impacted Files	Coverage Δ
extensions/omniv21/ingester.go	100.00% <ø> (ø)
extensions/omniv21/schemahandler.go	100.00% <ø> (ø)
extensions/omniv21/transform/decl.go	100.00% <ø> (ø)
extensions/omniv21/transform/invokeCustomParse.go	100.00% <ø> (ø)
extensions/omniv21/transform/parse.go	100.00% <ø> (ø)
extensions/omniv21/transform/validate.go	100.00% <ø> (ø)
validation/jsonvalidate.go	100.00% <ø> (ø)

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