chore: adjust usage of sst type

[ShiKaiWi]

Which issue does this PR close?

Prepare for #495

Rationale for this change

Refer to #495:

Currently, the sst type is used to decide which type of sst reader/builder to create, but this option is configured for table level, that is to say, one table can only use one sst type. I guess this is not reasonable, and it is more flexible and elegant to allow the data of one table being encoded in different sst formats. So I guess we make this option only works when deciding the type of sst builder, and add a new sst type called Auto for the flexibility of choosing a better sst format.

What changes are included in this PR?

• Remove sst type option when reading sst;
• Add a new sst type called Auto;

Are there any user-facing changes?

None.

How does this change test

Existing tests.

[chunshao90]

LGTM