Normalize `date` and `time` value annotations

[alancai98]

I noticed that some TIME values in the test data use an annotated struct:

e.g.: https://github.com/partiql/partiql-tests/blob/e24d8ef837303e675bc7c29b8a41f3cc81d6b96e/partiql-tests-data/eval/query/order-by.ion#L43

While some other TIME values use an annotated string:

e.g. https://github.com/partiql/partiql-tests/blob/e24d8ef837303e675bc7c29b8a41f3cc81d6b96e/partiql-tests-data/eval/spec-tests.ion#L755-L758

The partiql-tests spec proposal defines using the annotated string form: https://github.com/partiql/partiql-tests/blob/e24d8ef837303e675bc7c29b8a41f3cc81d6b96e/docs/partiql-tests-schema-proposal.md#modeling-partiql-types-in-ion.

These should likely be normalized to the same thing.

TODO:

• Also sort out the modeling of other time/timestamp types that use timezone

[alancai98]

date is also modeled using a timestamp in some tests

https://github.com/partiql/partiql-tests/blob/e24d8ef837303e675bc7c29b8a41f3cc81d6b96e/partiql-tests-data/eval/query/order-by.ion#L48

The spec proposal uses an annotated string: https://github.com/partiql/partiql-tests/blob/e24d8ef837303e675bc7c29b8a41f3cc81d6b96e/docs/partiql-tests-schema-proposal.md#modeling-partiql-types-in-ion.

[alancai98]

Also need to normalize how PartiQL timestamps are to be represented in Ion (could be the same as Ion?)