Feat/plan metadata

[arusahni]

Re-creating PR #16 because I had to adjust the build matrix, which ended up leaving phantom pending tasks that made it unmergable.

Orb now allows users to attach arbitrary key/value metadata to plans. Add support for this to the crate.

Additionally, clean up some tests:

• fix instances where we weren't actually interpolating locals
• temporarily work around Orb API regression re: None values in the events response
• retry the event amendment query a few times to account for variance in timing
• re-order the expected event list query results to match the query params order