avoid returning `&HashMap` on `channels_sv2` public APIs

[plebhash]

we should avoid returning &HashMap on channels_sv2 public APIs

we're exposing internal representation over public APIs, which makes the crate more prone to breaking changes on the long term, which could be avoidable

originally pointed out by @Shourya742 here: #2139 (comment)

aside from channels_sv2::client::share_accounting::ShareAccounting::get_rejected_shares (introduced via #2139), the methods that currently return &HashMap are:

• channels_sv2::client::group::GroupChannel::get_future_jobs
• channels_sv2::client::extended::ExtendedChannel::get_future_jobs
• channels_sv2::client::extended::ExtendedChannel::get_past_jobs
• channels_sv2::client::extended::ExtendedChannel::get_stale_jobs
• channels_sv2::client::standard::StandardChannel::get_future_jobs
• channels_sv2::client::standard::StandardChannel::get_past_jobs
• channels_sv2::client::standard::StandardChannel::get_stale_jobs

[plebhash]

while tackling this, we should also make these HashMaps into wrapper types for improved readability

#2139 (comment)