Add new COPY option REJECT_LIMIT

Hi,

9e2d870 enabled the COPY command to skip soft error, and I think we can 
add another option which specifies the maximum tolerable number of soft 
errors.

I remember this was discussed in [1], and feel it would be useful when 
loading 'dirty' data but there is a limit to how dirty it can be.

Attached a patch for this.

What do you think?


[1] 
https://www.postgresql.org/message-id/752672.1699474336%40sss.pgh.pa.us


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Regards,

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Atsushi Torikoshi
NTT DATA Group Corporation