Support for SCRAM-SHA-256 SASL authentication (vapor#89)

* Add protocol awareness and encode/decode for authentication message types 10 (SASL mechanisms), 11 (SASL continue), and 12 (SASL final). Add more specific errors for types 2(Kerberos), 7(GSSAPI), 8(GSSAPI), 9(SSPI), and 6(obsolete SCM).

* Add generic SASL authentication management class with pluggable (via generics) SASL mechanism implementations.

* A mostly complete, if very, VERY messy, implementation of SCRAM-SHA-256 and SCRAM-SHA-256-PLUS per RFC 7677 et al. Things that are still missing: Channel binding support (Postgres DOES use this), authorization names (Postgres does not use these), proper username and password normalization, RFC-compliant validation of nonces, and determining whether the Hi() function can be replaced with PBKDF2

* Extend PostgresConnection to use SCRAM-SHA-256 negotiation when offered.

* Heavily update test matrix. Leave several of the Swift version/OS combos disabled to cut down on the excessive number of checks generated by the test matrices (72 instead of 234).

Close connection if requestTLS fails (vapor#134)

* Close connection if requestTLS fails

* Add test to ensure we don't hit assert from leaking connection

Support custom JSON coders (vapor#127)

* Support non Foundation JSON coders

* add a `PostgresJSONEncoder` protocol with an API that mirrors the Foundation `JSONEncoder`'s decoding function

* add global `_defaultJSONEncoder` variable used in the JSON and JSONB type and that is defaulted to a Foundation `JSONEncoder`

* add a `PostgresJSONDecoder` protocol with an API that mirrors the Foundation `JSONDecoder`'s decoding function

* add global `_defaultJSONDecoder` variable used in the JSON and JSONB type and that is defaulted to a Foundation `JSONDecoder`

* Add docblocks for _defaultJSON{encoder,decoder} and their respective unit tests

Fix prepare(query:) when no data returned (vapor#123)

* Updated prepared query handling to work when no results are returned

* Added test for issue 122

Integer overflow fixes (vapor#120)

* int overflow fixes

* add new test

* reference pr

* update readme

Fix warnings + update CI (vapor#117)

* silence xcode 12b3 warnings

* update ci

* fix ci

* fix db name

support parameter status messages in query / simpleQuery (vapor#116)

Add support for VARCHAR[] (vapor#114)

* add varchar array type

* fix comment

Support remote close (vapor#110)

* support remote close

* fix fluent tests

* bump

* fix fluent test

make public