Make `penpal-runtime`'s `TrustedReserves` more connfigurable (#3564)

The current `penpal` runtime utilizes the `EthereumLocation` parameter,
which is employed for XCM emulated integration tests concerning the
Rococo <> ETH bridge. It includes a hard-coded chainId for the Ethereum
testnet utilized in Rococo. The `EthereumLocation` serves the purpose of
aligning with the `TrustedReserves`. However, due to this hard-coded
configuration, reusing `penpal` for testing various environments such as
Kusama/Polkadot versus Ethereum bridge becomes unfeasible.

This PR introduces the capability to easily customize the asset location
for `TrustedReserves` without needing to know anything about Ethereum.


## TODO
- [x] fix integration tests with
`System::set_storage(CustomizableAssetFromSystemAssetHub::key(),
<whatever-location-is-needed>)` @claravanstaden
- [ ] ~~maybe add some helper function/macro to support `set_storage`
for other runtimes (that we could reuse)~~
- [ ] Release patch for: `penpal-runtime` + emulated crate with
`set_storage` support (if needed)
  - [ ] backport to 1.7.0
  - [ ] backport to 1.8.0

---------

Co-authored-by: Clara van Staden <[email protected]>