Enhance Navigator's lf-value-json encoding to fully support DAML LF primitives

[leo-da]

Large Decimal values is one of the cases.
See #2519

[S11001001]

banker's rounding

In most contexts, we expect LF values to have been pre-rounded, and don't do rounding again in order to avoid the risk of discarding data. That includes the Time.Timestamp LF Scala type underlying the LF Value ADT.

For the JSON format, we state:

Numbers must be within the bounds of Decimal, [–(10³⁸–1)÷10¹⁰, (10³⁸–1)÷10¹⁰]. Numbers outside those bounds will be rejected. Numbers inside the bounds will always be accepted, using banker's rounding to fit them within the precision supported by Decimal.

[S11001001]

non-Z timestamps

Hypothetically, we could read timezones in the timestamp format. Instead, we only permit Z, and moreover require that Z is specified.

The UTC timezone designator must be included. The rationale behind the inclusion of the timezone designator is minimizing the risk that users pass in local times.

[S11001001]

nested Optional

The Optional rules provide a consistent core rule for nested optionals:

1. the innermost Optional (i.e. any optional without an optional directly therein) doesn't have a marker. null or absence (in the case of a surrounding [] for containing optional) means None.
2. [] is a valid Some None.

This is the ruleset for the Optional section, but some other encodings assume something incompatible. For example, [42] is never a valid Optional Int under this scheme, nor is [] a valid None.