Get each field of default resources from tar_option_get("resources") separately

[wlandau]

Prework

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Description

At the time of writing, target resources need to be supplied wholesale. That is, if you set tar_option_set(resources = tar_resources(aws = tar_resources_aws(bucket = a, endpoint = b), feather = c)) and then tar_target(x, .., resources = tar_resources(aws = tar_resources_aws(bucket = d)), the feather resource of target x will not be b and the AWS endpoint of target x will not be b. I have not heard any reports of confusion from this, but it could easily create confusion. The defaults of all tar_resources() functions should come from tar_option_set("resources")

[wlandau]

In targets version 0.12.2 and above, resources will be inherited one-by-one in nested fashion from tar_option_get("resources").For example, suppose you set tar_option_set(resources = tar_resources(aws = my_aws)), where my_aws equals tar_resources_aws(bucket = "x", prefix = "y").Then, tar_target(data, get_data() will have bucket "x" and prefix "y". In addition, if new_resources equals tar_resources(aws = tar_resources_aws(bucket = "z"))), then tar_target(data, get_data(), resources = new_resources) will use the new bucket "z", but it will still use the prefix "y" supplied through tar_option_set(). (In targets 0.12.1 and below, options like prefix do not carry over from tar_option_set() if you supply non-default resources to tar_target().)

[wlandau]

I believe this change will eliminate potential confusion and pitfalls because of how deeply nested the resources option is. I am planning to allow an option for a scratch directory to be declared this way (#889, #891) so users may be relying on it more, which makes it extra important to make the default behavior agree with prior expectation.