The W3C spec specifies that values must be percent-encoded so when
reading the environment variable `OTEL_RESOURCE_ATTRIBUTES` the SDK
should decode them.

This is done by the `baggage` package, but its behaviour in case of
errors is slightly different from the current implementation of the SDK,
more specifically in cases where a key is missing a value. The SDK
returns a partial resource while the `bagage` package returns nil.

This may be considered a breaking change, so this commit fixes the
current implementation instead of using `baggage.Parse`.

…e-atttributes

…e-atttributes