lib-sieve: variables extension: Migrate settings to new config structure

src/lib-sieve/plugins/variables/Makefile.am

src/lib-sieve/plugins/variables/ext-variables-common.c

src/lib-sieve/plugins/variables/ext-variables-common.h

src/lib-sieve/plugins/variables/ext-variables-limits.h

src/lib-sieve/plugins/variables/ext-variables-modifiers.c

src/lib-sieve/plugins/variables/ext-variables-operands.c

src/lib-sieve/plugins/variables/ext-variables-settings.c

src/lib-sieve/plugins/variables/ext-variables-settings.h