Added validations on keywords & identifiers for search catalog_items API

[amoludage]

Current Behaviour -

• For SearchCatalogItems API, if we send keywords and identifiers both the values then we calling SPAPI and getting error from that API that is Invalid combination of 'identifiers' and 'keywords' provided. This is because the API support either of these values and not both.
• Also, if we don't send any of these two values to the API then we are getting error from SPAPI that is Missing required 'identifiers' or 'keywords'.

Problem Statement -

• This gem should avoid such API calls to SPAPI if the expected parameters are missing.

Proposed Solution -

• We are adding validations on keywords and identifiers in this gem and avoid SPAPI API call and return error from the gem only.