chore: initialize mito2 crate

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR is a part of #1869. It initializes a new crate mito2 to experiment with refactoring the old mito engine to a region engine.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

• Tracking issue for implementing mito as a region engine #1869

[codecov]

Codecov Report

Merging #1875 (5b4fc8a) into develop (2ef84f6) will decrease coverage by 0.34%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #1875      +/-   ##
===========================================
- Coverage    86.50%   86.17%   -0.34%     
===========================================
  Files          593      597       +4     
  Lines        96040    97142    +1102     
===========================================
+ Hits         83083    83711     +628     
- Misses       12957    13431     +474     

[v0y4g3r]

LGTM