Timestamp Addition Inconsistencies

There are two ways of calling performing timestamp addition in pinot:
1. `TIMESTAMPADD`
2. `DATEADD`

The former is parsed directly by Calcite while the latter is passed into the Pinot engine. This leads to a few issues:
1. `TIMESTAMPADD` expects a time interval literal while `DATEADD` expects a string as the first argument. This means that `TIMESTAMPADD('DAY', ...)` is invalid and should be `TIMESTAMPADD(DAY, ...` while `DATEADD(DAY, ...)` is invalid and should be `DATEADD('DAY', ...`
2. Calcite parses the first argument as a `TimestampInterval()`, which does not include some units that are supported in `DATEADD` (such as, notably, `MILLISECOND`) which means that `TIMESTAMPADD(MILLISECOND` fails to parse while `DATEADD('MILLISECOND'` succeeds

Start by tracing how TIMESTAMPADD is parsed by Calcite and how DATEADD is handled in the Pinot engine. Compare their argument formats and supported time units, especially MILLISECOND; done means the two timestamp-addition functions have consistent, documented parsing and unit behavior.

Written by the indexing model from the issue text.