[MetaSchedule] Developer Ergonomics Enhancement

[junrushao]

Per discussion with @Kathryn-cat:

• Encourage using TuneContext.initialize, and moving initialize_with_tune_context as private
  API _initialize_with_tune_context;
• Encourage using ms.xxx as the prefix (e.g. ms.database.MemoryDatabase) to organize code
  better, instead of using multiple import statements;
• Encourage using ms.default_config.xxx (e.g. schedule_rule);
• Move tvm.meta_schedule.testing.DummyDatabase to tvm.meta_schedule.database.MemoryDatabase
  given it's actually functioning;
• Delegate class members' methods in TuneContext, for example, having
  TuneContext.generste_design_space from TuneContext.space_generator.generste_design_space

Next PR:

• Allow using a string "default" in TuneContext as well as tune_relay/tir/te to quickly
  specify a set of target-specific rules
• Add TuneContext.tune to allow directly tuning without task scheduler.
• Enhance detection of ScheduleFn in TuneContext to make it easier for users to quickly try out
  template-driven scheduling on TIR.

[junrushao]

To avoid the PR being overly huge, we decided to upstream the rest of the enhancement as a separate PR.

[zxybazh]

The changes makes a lot of sense to me and generally making it easier to access each function in the task. Generally looks good to me. A couple nits as mentioned in Kathryn's comments could further simplify the functions. It looks to me that SendtoBuilder and SendtoRunner could be further put into TuneContext level but let's leave that to the next PR. Thanks for the changes!