Move IasAVRRequest struct from ias_proxy to common place of ias_client and ias_proxy

[manojgop]

IasClient talks to IAS or IAS proxy, we do not know behavior of IAS when empty values are passed with keys (PSE manifest and nonce). If this is acceptable by IAS replace the hashmap in POST request in ias_client to struct.

Note: There's already a struct in ias_proxy, but since it only uses it to deserialize request we do not need hashmap in ias_proxy. IAS proxy doesn't construct the request, it just forwards whatever is received to IAS.

Reason for deserializing request in IAS proxy is to get quote from it, which is used to store in LRU cache.