Context variables clear/re-initialization support

[EPluribusUnum]

Hi *!

Currently it can be made 2 ways.

1. SELECT rdb$set_context('USER_SESSION', v.mon$variable_name, NULL) FROM mon$context_variables
2. ALTER SESSION RESET

Both solution is slow and has issues

1. mon$ read is expensive; blob usage
2. does more work than needed; ends transaction

@hvlad suggested RDB$RESET_CONTEXT(namespace) system function in this firebird-devel conversation:
https://groups.google.com/g/firebird-devel/c/ttJtZe4zUUM/m/zrRpEiUhBAAJ

Thank you!