pgsql: Expose helper functions scan_quoted_identifier and scan_identifi

Expose helper functions scan_quoted_identifier and scan_identifier.

Previously, this logic was embedded within SplitIdentifierString,
SplitDirectoriesString, and SplitGUCList. Factoring it out saves
a bit of duplicated code, and also makes it available to extensions
that might want to do similar things without necessarily wanting to
do exactly the same thing.

Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Lukas Fittl <lukas@fittl.com>
Discussion: http://postgr.es/m/CA+Tgmob-0W8306mvrJX5Urtqt1AAasu8pi4yLrZ1XfwZU-Uj1w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0e819cc08d3dc6fe843779c2bb3388135d20d14

Modified Files
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src/backend/utils/adt/varlena.c | 189 +++++++++++++++++++++-------------------
src/include/utils/varlena.h     |   3 +
2 files changed, 102 insertions(+), 90 deletions(-)