EQL: implement concat function

[rw-access]

Part of #51556

Call toString() on all the non-null input arguments.

For the null behavior, we could go one of three ways:

• if any of the inputs are null, return null
• if all of the inputs are null, return null
• return an empty string, if all inputs are null

Update from 2020/04/23:

• We decided to go with the first option. All inputs are required, so if any are missing, we will return null. This will be more consistent with other functions, and make the strings within concat more consistently parseable.

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/EQL)