configuration value fetching is inefficient

[SwooshyCueb]

At present, in storage_tiering_configuration.cpp, for each configuration value, we first check for the existence of the value with nlohmann::basic_json::find, before fetching the value with nlohmann::basic_json::at, like so:

irods_capability_storage_tiering/storage_tiering_configuration.cpp

Lines 23 to 26 in 7ec9561

	if(plugin_spec_cfg.find("access_time_attribute") != plugin_spec_cfg.end()) {
	access_time_attribute = boost::any_cast<std::string>(
	plugin_spec_cfg.at("access_time_attribute"));
	}

To my understanding, this is inefficient.
The return value of nlohmann::basic_json::find is an iterator, which will already contain the value we're looking for (or a reference to it) if it was found, but we throw it away and find the value again with nlohmann::basic_json::at.

See irods_server_properties.hpp for an example of proper usage of nlohmann::basic_json::find:
https://github.com/irods/irods/blob/84a09f945899b5ad85bb3d4cc451812e9dc05358/lib/core/include/irods_server_properties.hpp#L68-L71