_OBS_Precheck and OBS_MetadataValidator

New functions to perform a precheck for the user data and metadata
parameters that are going to be used to execute a OBS_GetData operation.

Doing this before the OBD_GetData we are able to check if we're going to
have problems with the current request. Is useful because right now
we're using the *_Exception_Safe functions which return NULL in case we
have an exception, this leads to execute the whole request even if we
know beforehand that it's going to fail.

server/extension/test/sql/100_data_observatory_test.sql

@@ -172,3 +172,10 @@ SELECT exists(SELECT *
               WHERE ns.nspname = 'cdb_dataservices_server'
               AND proname = 'obs_legacybuildermetadata'
               AND oidvectortypes(p.proargtypes)  = 'text, text, text');
+
+SELECT exists(SELECT *
+              FROM pg_proc p
+              INNER JOIN pg_namespace ns ON (p.pronamespace = ns.oid)
+              WHERE ns.nspname = 'cdb_dataservices_server'
+              AND proname = 'obs_metadatavalidation'
+              AND oidvectortypes(p.proargtypes)  = 'text, text, geometry, text, json, integer');