lua: add tweaks module

This commit adds the new 'internal.tweaks' Lua module for toggling
internal tweaks. The module is implemented as a Lua table with the
__index and __newindex metamethods overridden, which makes it possible
to get/set a tweak value with a simple assignment, for example:

  tarantool> tweaks = require('internal.tweaks')
  ---
  ...

  tarantool> tweaks.fiber_channel_close_mode = 'graceful'
  ---
  ...

  tarantool> tweaks.fiber_channel_close_mode
  ---
  - graceful
  ...

(Note, currently there are no tweaks available; tweaks are added in the
following commits.)

Getting a value of an unknown tweak returns nil. Setting a value of an
unknown tweak raises 'No such option' error. Setting an invalid value
for an existing tweak raises 'Invalid value' error. The module also
implements __serialize and __autocomplete metamethods that return all
tweaks and their values in a table, for example:

  tarantool> tweaks
  ---
  - fiber_channel_close_mode: forceful
    yaml_pretty_multiline: false
    json_escape_forward_slash_default: true
  ...

Closes #7883
Needed for #8117

NO_DOC=internal
NO_CHANGELOG=internal