[Pre-1.0] Convert Attributes to a KV API like Go's std context, but without the deadlines and timeouts

[abraithwaite]

Currently Attributes act like a linked-list but this creates challenges when adding to or looking up values in the list.

Does someone implementing attributes have to consider if there's another type further down the list and continue? If so, which is the valid one if the 2 attributes in the chain have conflicting values?

Overall, it's a very useful API but could use improvement by modeling context's key-value nature allowing for setting keys independently of one another, unlike the linked-list like implementation.