FR: Detect contracts outside the `contracts/` directory using `Cargo.toml`

## Description

Currently, the [optimizer assumes](https://github.com/CosmWasm/optimizer/blob/de8e0bf7be26101327caa55c44f029073104de06/bob_the_builder/src/lib.rs#L64) that all contracts in a workspace are located in the contracts/ directory. However, this is not always the case, especially with larger libraries such as [ibc-rs](https://github.com/cosmos/ibc-rs). For instance, `ibc-rs` has its `07-tendermint` contract in `./ibc-clients/ics07-tendermint/cw-contract`, which is not detected by the optimizer. It would be beneficial to define a build process that specifies the contract packages in `Cargo.toml`.

## Proposal: Look for `[package.metadata.optimizer]`

Currently, the optimizer only iterates over `Cargo.toml` files of the packages in the contracts/ directory. While retaining this behavior, extend the optimizer to iterate over all `Cargo.toml` files of all packages in the workspace and treat the packages that have `[package.metadata.optimizer]` as contracts to be built.

This enhancement will provide greater flexibility and support for projects with different directory structures, ensuring all relevant contracts are detected and optimized.