Implement From for AnyPool

[prabirshrestha]

Is your feature request related to a problem? Please describe.
Most sqlx libs doesn't seem to implement AnyPool but rather SqlitePool or PgPool. This makes my code which use AnyPool incompatible with the external libraries.

Describe the solution you'd like
From is implemented for AnyConnection. Is it possible to do the same for AnyPool? Would also be good to have try_into() so AnyPool can be can be converted to SqlitePool, PgPool or error out.

Describe alternatives you've considered
Need to use Pool so Connection alone is not enough.

Additional context
Example use case: ZeroAssumptions/aide-de-camp#11, casbin-rs/sqlx-adapter#77. It wouldn't scale to ask every libraries to support AnyPool.