docs: spec plan of tgdump with any SQL statements.

[ashigeru]

This PR proposes a design that introduces the --sql option to the tgdump command to allow dump processing by any SQL statement.
Note that only the documentation will be changed at this time, and implementation will be done later if this proposal is accepted.

[akirakw]

LGTM.
However, the handling of --sql and <query-label> is a bit complicated, so it would be better to provide command examples.

[hishidama]

特に問題無いと思います。

（ドキュメントに書くことではないと思いますが、実際の使い方は --sql "sql1: select ～" のようにコロンの前後に空白が入ることが予想されるので、それが許容されるのであれば問題ないと思います）

[ashigeru]

@akirakw #44 (review)

LGTM. However, the handling of --sql and <query-label> is a bit complicated, so it would be better to provide command examples.

We plan to revise it later with another PR.