Allow tables to handle _id values in non DS+ rows

[aptkingston]

Description

This PR allows the usage of existing values for an _id column on non-datasource-plus sources. An example is Mongo, which has it's own concept of _id separate to Budibases. The current table component assumes that non DS+ sources will not contain this field, and therefore overwrites it with a UUID in order to be able to uniquely identify each row.

This PR will now use existing _id values if they exist, trusting that they are unique, and continuing to generate them if not.

I've also updated the additional context we pull from tables to better reflect the available data. This means that fields like _id and __idx which the table generates for internal use will not appear in live binding evaluation.

Assumptions

The grid has 2 remaining assumptions.

• _id values in non DS+ rows are unique
• _id values in non DS+ rows do not start with ‽‽ (that's not a question mark, it's an obscure character chosen since it's extremely rare and will never be used)

Addresses

A customer issue.

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[deanhannigan]

LGTM!