Add individual calls (simplex, duplex, over Brew)

[proxiboi69]

Adds individual (point-to-point) calls: local simplex/duplex, off-cell/PBX via Brew (both directions), and long external-number dialing.

• Simplex + duplex calls, hook and direct setup
• Off-cell/PBX individual calls over Brew, both directions
• External subscriber number decode beyond 16 digits
• ETSI EN 300 392-2 clause 14.5.1 compliant, verified against spec and Brew protocol docs

[retsplines]

Thanks for tackling this, huge bit of work 🩶 will definitely check it out over the weekend.

I made a start on this myself but ended up ripping up a bunch of floorboards that weren't easy to put back.

I think cc_bs.rs is getting very big for it's boots and needs a refactor at this stage, but I conceded defeat on that. I started working it into a set of state machines for each call type. I also bumped into the issue of the actual routing of traffic and started to consider some architectural changes like bringing the handling of TMD-SAP messages into the CMCE rather than doing the call bridging at the UMAC.

[wbokslag]

Thanks for the PR, will test soon. @retsplines , agree on the needed refactor on cc_bs.rs, I think we'll need to improve state management and make a true, enforced state machine instead of scattering bits and pieces all over the stack. If this PR works however, I'd say we merge first, and put that additional refactoring work on the roadmap for some point in the future.
I'll test soon and get back, interested in your results as well