Add --properties option to seqcli ingest and seqcli log

[nblumhardt]

Currently, seqcli log and seqcli ingest allow additional properties to be specified in -p Name=Value format:

seqcli log -m "Hello, {Name}" -p Name=User

This relies on sniffing the type of the property value to construct string, numeric, or Boolean property values in a reasonably intuitive manner.

However, there's no way to override the type used for the representation of a value, and no way to specify complex values such as service: {name: 'Test'} or tags: [1, 2, 3].

This PR is a proof-of-concept showing how we could use Seq.Syntax to construct strongly-typed property values:

seqcli log -m "Hello, {Name}" --properties="{Name: 'User', Tags: [1, 2, 3]}"

Each member in the result of evaluating --properties is added to the log event. A tricky "object spread" is used behind the scenes so that in the seqcli ingest case, properties on the original event being ingested can be removed with {someProperty: undefined()}.

Posting this here as a spike; some more work is needed on tests, and there's a bit of an unpleasant split in seqcli log due to the current implementation no longer relying on LogEvent behind the scenes.