scd2 extension: active record literal changes

[jorritsandbrink]

Description

active record literal = literal used to indicate active records in scd2 tables

This PR

• changes default active record literal from a high timestamp to NULL
• introduces active_record_timestamp setting that gives users control over the active record literal
  • accepts None (default) or an object conforming to TAnyDateTime

@dlt.resource(
    ...,
    write_disposition={
        "disposition": "merge",
        "strategy": "scd2",
        "active_record_timestamp": "9999-12-31",  # e.g. `datetime` is also okay
    },
    ...
)

Related Issues

Partially implements #1252

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[sh-rp]

Very nice, thanks :) We just need the docs for it now, then we can merge!

[sh-rp]

Merging since tests passed on prev commit, this was just a docs change.