fix: parsing time stamps with time zone offsets containing colons

[toph-allen]

Fixes #290

Intent

Fix a bug involving the handling of timestamps from Connect.

Approach

Connect sends timestamps in RFC 3339 format. Specifically:

• Timestamps with no UTC offset end in Z, e.g. "2020-01-01T01:02:03Z"
• Timestamps with an offset end in ±hh:mm, e.g. "2023-08-22T15:13:14+01:00"

Details on the approaches used.

• This PR fixes a bug where we would not parse time stamps with an offset, resulting in NA values. It uses a combination of regular expressions and using different time format specifiers for different incoming time stamps.
• Converts all incoming timestamps to UTC. Previously, we only parsed UTC times at all, but just fixing the time format revealed a bug where we assigned UTC to non-UTC timestamps.
• Fixes a bug in make_timestamp(), which creates timestamps for Connect. Previously, would output all timestamps as UTC, even though they were in the local system time.

Tests

Added tests for a range of expected inputs. Tests take place in multiple time zones.

[tdstein]

Nicely done! Thanks for all of the comments. Do we need to verify that dates roll over correctly from different time zones into UTC?

[aronatkins]

Lacks NEWS update. Approval assumes addition of NEWS.