[border-agent] update CoapDtlsSession to handle session functionali…

…ty (openthread#11131)

This commit updates the `CoapDtlsSession` class in `BorderAgent` to
move all session-related functionality into this class. It now
handles its own state and uses its own `Timer`, ALOC, UDP receiver,
etc.

Many methods previously defined in `BorderAgent` are now defined in
`CoapDtlsSession`, ensuring each session can be processed
individually. The `ForwardContext`, which tracks forwarded TMF
commands, is also moved into `CoapDtlsSession` and updated to track
its associated session.

This change prepares `BorderAgent` to support multiple concurrent
sessions in future PRs.