fix(bitcoind_rpc): allow filter_iter example to compile on Windows

fix(bitcoind_rpc): allow filter_iter example to compile on Windows

The filter_iter example previously imported anyhow indirectly from
bdk_testenv. Because Cargo includes package dev-dependencies when
building examples, having bdk_testenv as an unconditional
dev-dependency pulled in electrsd, whose build script relies on Unix
permissions (std::os::unix::fs::PermissionsExt) and fails on Windows.

This change:
- Adds anyhow directly to [dev-dependencies] for bdk_bitcoind_rpc.
- Removes the indirect bdk_testenv::anyhow import in filter_iter.rs.
- Moves bdk_testenv to [target.'cfg(not(windows))'.dev-dependencies].
- Conditionally gates the bdk_testenv-dependent tests on not(windows).

Closes #2338.