[processor/transform] Extract function arguments into separate object…

[anuraaga]

… for easier extension

Description: Extracts arguments passed to transform functions into a struct for easy extension

@Aneurysm9 had originally had this idea and initially I went with simpler listing out the arguments to avoid any performance issues. Because we call functions through function pointers, Go can't really optimize the struct away. That being said, being able to extend the data that is available to a transform function also seems important, for example I heard about a case to provide auth credentials information to transformation. So I want to see what people think of this style.

The benchmarks are consistently slower but for 100 spans ranges from 2-15us or so, getting consistent numbers at this scale is difficult so it seems to still be within noise to some extent. Let me know thoughts.

Link to tracking Issue: #7300

Testing: Unit tests

Documentation:

After

BenchmarkTwoSpans/no_processing-16                777204              1487 ns/op
BenchmarkTwoSpans/set_attribute-16                720290              1669 ns/op
BenchmarkTwoSpans/keep_keys_attribute-16          654525              1865 ns/op
BenchmarkTwoSpans/no_match-16                     749661              1600 ns/op
BenchmarkTwoSpans/inner_field-16                  724689              1667 ns/op
BenchmarkTwoSpans/inner_field_both_spans-16               696908              1742 ns/op
BenchmarkHundredSpans/no_processing-16                     25939             46176 ns/op
BenchmarkHundredSpans/set_status_code-16                   21024             57097 ns/op
BenchmarkHundredSpans/hundred_queries-16                    2462            497112 ns/op

Before

BenchmarkTwoSpans/no_processing-16                766363              1497 ns/op
BenchmarkTwoSpans/set_attribute-16                712767              1675 ns/op
BenchmarkTwoSpans/keep_keys_attribute-16          647737              1885 ns/op
BenchmarkTwoSpans/no_match-16                     756135              1596 ns/op
BenchmarkTwoSpans/inner_field-16                  730369              1661 ns/op
BenchmarkTwoSpans/inner_field_both_spans-16               690099              1735 ns/op
BenchmarkHundredSpans/no_processing-16                     25314             46236 ns/op
BenchmarkHundredSpans/set_status_code-16                   21282             56543 ns/op
BenchmarkHundredSpans/hundred_queries-16                    2520            473097 ns/op

[Aneurysm9]

Would be nice to include before/after benchmarks in the description for posterity, but this LGTM.