sql/types: implement NumberType for system numeric types

[ym0506]

Summary

• Implement sql.NumberType for system numeric system-variable types:
  • SystemBoolType
  • systemIntType
  • systemUintType
  • systemDoubleType
• Add compile-time interface conformance checks for these types.
• Add tests to verify:
  • sql.IsNumberType recognizes system numeric types.
  • stats join alignment accepts system numeric types (AlignBuckets path).

Testing

• go test ./sql/types ./sql/stats
• go test ./sql/... -run TestDoesNotExist -tags=gms_pure_go

Related

• Relates to Implement sql type interfaces for system types dolt#10535

[angelamayxie]

Please see the comments I left. Also, I removed the mention in the description to the join issue -- this PR doesn't actually add any relevant testing.

[ym0506]

Thanks for the review — addressed in b0e739b !!

• Removed the join test from stats since it was not directly relevant.
• Moved the sql type-interface checks into a dedicated test file:
  sql/types/system_types_test.go

Please let me know if you prefer a different test organization.

[angelamayxie]

LGTM! Thanks for the contribution :) I'll go ahead and merge it for you